AP_ExternalAHRS: Compare FIX_TYPE judgment with definition name

[muramura]

I found GPS_FIX_TYPE compared to the direct value.
This value is defined.
I changed it to compare by definition name.

[peterbarker]

This seems reasonable in that we're assigning from an enumeration elsewhere.

This doesn't seem reasonable in that we shouldn't be using a mavlink enumeration in here. We have AP_GPS::GPS_Status which could be used instead.

[khancyr]

@peterbarker in fact the PR is correct, the driver already make use of mavlink enum for GPS and EKF. So I would say , let's just merge this and open an issue to remove the Mavlink dependencies

[peterbarker]

I've created #28254 which doesn't extend our coupling with mavlink instead.