Copter: use disparity threshold define for pre-arm checks

There are two duplicate checks, one in the pre-arm checks (i.e. checks
run every 15 seconds or so before the vehicle is armed) and one in the
arming checks (run immediately before arming).  The definition in the
pre-arm checks was still using the old hardcoded value.
Compass: always default devid to zero
Copter: allow passthru for ch 9 ~ 14

Based on work by Emile Castelnuovo
Merge remote-tracking branch 'upstream/master'