[AX25] Added check for Bell modem

src/protocols/AX25/AX25.cpp

@@ -194,8 +194,13 @@ int16_t AX25Client::begin(const char* srcCallsign, uint8_t srcSSID, uint8_t preL
   preambleLen = preLen;
 
   // configure for direct mode
+  #if !defined(RADIOLIB_EXCLUDE_AFSK)
+  if(bellModem != nullptr) {
     return(phyLayer->startDirect());
   }
+  #endif
+  return(RADIOLIB_ERR_NONE);
+}
 
 #if defined(RADIOLIB_BUILD_ARDUINO)
 int16_t AX25Client::transmit(String& str, const char* destCallsign, uint8_t destSSID) {