Cast enum to int to fix Qurt compiler warning

[katzfey]

Cast enum to an int to get rid of Qurt compiler warning when max enum value is less than the constant it is being compared to. Here is the warning:

../../libraries/AP_GPS/AP_GPS.cpp:1622:14: warning: result of comparison of constant 8 with expression of type 'mavlink_channel_t' is always false [-Wtautological-constant-out-of-range-compare]
if (chan >= MAVLINK_COMM_NUM_BUFFERS) {
~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~

[peterbarker]

We went the other way for a similar thing:

libraries/GCS_MAVLink/GCS_MAVLink.h:#pragma clang diagnostic ignored "-Wtautological-constant-out-of-range-compare"