ath: clean up ath_reg_apply_active_scan_flags()

The routine ath_reg_apply_active_scan_flags() can be a bit
hard to read, this cleans it up by adding helpers for the
two cases of clearing IR flags or adding them. This approach
also makes no assumptions on the index of channels 12 and 13
so it should be portable accross different drivers.

Cc: smihir@qti.qualcomm.com
Cc: tushnimb@qca.qualcomm.com
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>