apps/modbus/mb_m.c: missing eMBMasterASCIIStart definition

Start in apps/modbus/mb_m.c at eMBMasterInit and inspect the MB_ASCII_MASTER branch and its referenced ASCII callbacks. Build with MB_ASCII_MASTER enabled, then verify that the missing ASCII symbols and the unhandled master events no longer produce the reported compiler diagnostics.

(This issue was reported by user @klmchp at incubator-nuttx repository)

Error message as shown below when enabled MB_ASCII_MASTER:
mb_m.c: In function 'eMBMasterInit':
mb_m.c:176:33: error: 'eMBMasterASCIIStart' undeclared (first use in this function); did you mean 'eMBMasterRTUStart'?
176 | pvMBMasterFrameStartCur = eMBMasterASCIIStart;
| ^~~~~~~~~~~~~~~~~~~
| eMBMasterRTUStart
mb_m.c:176:33: note: each undeclared identifier is reported only once for each function it appears in
mb_m.c:177:32: error: 'eMBMasterASCIIStop' undeclared (first use in this function); did you mean 'eMBMasterRTUStop'?
177 | pvMBMasterFrameStopCur = eMBMasterASCIIStop;
| ^~~~~~~~~~~~~~~~~~
| eMBMasterRTUStop
mb_m.c:178:32: error: 'eMBMasterASCIISend' undeclared (first use in this function); did you mean 'eMBMasterRTUSend'?
178 | peMBMasterFrameSendCur = eMBMasterASCIISend;
| ^~~~~~~~~~~~~~~~~~
| eMBMasterRTUSend
mb_m.c:179:35: error: 'eMBMasterASCIIReceive' undeclared (first use in this function); did you mean 'eMBMasterRTUReceive'?
179 | peMBMasterFrameReceiveCur = eMBMasterASCIIReceive;
| ^~~~~~~~~~~~~~~~~~~~~
| eMBMasterRTUReceive
mb_m.c:181:39: error: 'xMBMasterASCIIReceiveFSM' undeclared (first use in this function); did you mean 'xMBMasterRTUReceiveFSM'?
181 | pxMBMasterFrameCBByteReceived = xMBMasterASCIIReceiveFSM;
| ^~~~~~~~~~~~~~~~~~~~~~~~
| xMBMasterRTUReceiveFSM
mb_m.c:182:43: error: 'xMBMasterASCIITransmitFSM' undeclared (first use in this function); did you mean 'xMBMasterRTUTransmitFSM'?
182 | pxMBMasterFrameCBTransmitterEmpty = xMBMasterASCIITransmitFSM;
| ^~~~~~~~~~~~~~~~~~~~~~~~~
| xMBMasterRTUTransmitFSM
mb_m.c:183:38: error: 'xMBMasterASCIITimerT1SExpired' undeclared (first use in this function); did you mean 'xMBASCIITimerT1SExpired'?
183 | pxMBMasterPortCBTimerExpired = xMBMasterASCIITimerT1SExpired;
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| xMBASCIITimerT1SExpired
mb_m.c:185:17: warning: implicit declaration of function 'eMBMasterASCIIInit'; did you mean 'eMBMasterTCPInit'? [-Wimplicit-function-declaration]
185 | eStatus = eMBMasterASCIIInit(ucPort, ulBaudRate, eParity);
| ^~~~~~~~~~~~~~~~~~
| eMBMasterTCPInit
mb_m.c: In function 'eMBMasterPoll':
mb_m.c:301:7: warning: enumeration value 'EV_MASTER_PROCESS_SUCCESS' not handled in switch [-Wswitch]
301 | switch (eEvent)
| ^~~~~~
mb_m.c:301:7: warning: enumeration value 'EV_MASTER_ERROR_RESPOND_TIMEOUT' not handled in switch [-Wswitch]
mb_m.c:301:7: warning: enumeration value 'EV_MASTER_ERROR_RECEIVE_DATA' not handled in switch [-Wswitch]
mb_m.c:301:7: warning: enumeration value 'EV_MASTER_ERROR_EXECUTE_FUNCTION' not handled in switch [-Wswitch]