GCS_MAVLink: convert to using hal.serial() instead of hal.uartX

libraries/GCS_MAVLink/GCS.cpp

@@ -110,7 +110,7 @@ void GCS::send_named_float(const char *name, float value) const
 /*
   install an alternative protocol handler. This allows another
   protocol to take over the link if MAVLink goes idle. It is used to
-  allow for the AP_BLHeli pass-thru protocols to run on hal.uartA
+  allow for the AP_BLHeli pass-thru protocols to run on hal.serial(0)
  */
 bool GCS::install_alternative_protocol(mavlink_channel_t c, GCS_MAVLINK::protocol_handler_fn_t handler)
 {

libraries/GCS_MAVLink/GCS_serial_control.cpp

@@ -62,11 +62,11 @@ void GCS_MAVLINK::handle_serial_control(const mavlink_message_t &msg)
         break;
     }
     case SERIAL_CONTROL_DEV_GPS1:
-        stream = port = hal.uartB;
+        stream = port = hal.serial(3);
         AP::gps().lock_port(0, exclusive);
         break;
     case SERIAL_CONTROL_DEV_GPS2:
-        stream = port = hal.uartE;
+        stream = port = hal.serial(4);
         AP::gps().lock_port(1, exclusive);
         break;
     case SERIAL_CONTROL_DEV_SHELL: