Cellular: Fix stop tag for Quectel M26 send command

Possible responses for send command are SEND OK<cr><ln>, SEND FAIL<cr><ln> or ERROR<cr><ln>
so normal OK<cr><ln> response check does not work properly.

features/cellular/framework/targets/QUECTEL/M26/QUECTEL_M26_CellularStack.cpp

@@ -439,6 +439,7 @@ nsapi_size_or_error_t QUECTEL_M26_CellularStack::socket_sendto_impl(CellularSock
     _at.resp_start(">");
     _at.write_bytes((uint8_t *)data, sent_len);
     _at.resp_start();
+    _at.set_stop_tag("\r\n");
     _at.resp_stop();
 
     if (_at.get_last_error() != NSAPI_ERROR_OK) {