GCS_MAVLink: Remove pragma to disable compiler warning and instead ca…

…st enum to int to fix the code.

libraries/GCS_MAVLink/GCS_MAVLink.h

@@ -55,10 +55,7 @@ extern mavlink_system_t mavlink_system;
 /// @param chan		Channel to send to
 static inline bool valid_channel(mavlink_channel_t chan)
 {
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wtautological-constant-out-of-range-compare"
-    return chan < MAVLINK_COMM_NUM_BUFFERS;
-#pragma clang diagnostic pop
+    return static_cast<int>(chan) < MAVLINK_COMM_NUM_BUFFERS;
 }
 
 mavlink_message_t* mavlink_get_channel_buffer(uint8_t chan);