ao_flight: ao_interval_end type was too small, could cause premature landing state

ao_interval_end is used to provide a window of time during which the
max/min values for sensors are measured. After that time expires,
those bounds are compared to see if the airframe has been stable and
should be moved to landing state. With a type that is too small, that
could happen immediately after transitioning to main (for baro-enabled
devices) or coast (for accel-only devices). For baro devices, this
would disable any redundant main firing events. For accel-only
devices, this could disable all events occuring after coast, including
firing separation charges or motor igniters.

There are also a couple of additional changes to reduce -Wconversion
messages.

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