ath9k: separate ath9k specific code from ath9k_regd_get_ctl()

Until ath5k and ath9k share common channel structures, they will have
to implement their own get_ctl() function.  Split out the portion that
only relies on the current band and reg domain so that it can be common
code.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>