Add error checking around ping squawk value to avoid lockup

[rmackay9]

We have a report that MP locksup if and ADSB ping's "squawk" value is set to be out of range (e.g. 1200 and 7777). We should add better error checking around what the user sets this to to avoid the lockup.

[meee1]

root cause is message mavlink_uavionix_adsb_out_status_t is sending xpdr_mode_A_squawk_code = status.squawk; as 10110

which is out or range from 7777 to 1200