altos: Sort out ao_gps_print altitude fetching

ao_gps_print is used by both teledongle/telebt and the host-based GPS
test code. The first instance uses the old internal GPS structure,
containing just a 16-bit altitude while the second uses an
ao_telemetry structure, which contains 32 bits split into two
members.

Signed-off-by: Keith Packard <keithp@keithp.com>