ath: refactor/split getchannels() method.

Split getchannels() method in ath_hal/ah_regdomain.c into a subset
of functions for better readability.

Note: due to different internal structure, it cannot use
ieee80211_add_channel*() (however, some parts are done in a
similar manner).

Differential Revision: https://reviews.freebsd.org/D6139