ath: Add function to check if 4.9GHz channels are allowed

ath: Add function to check if 4.9GHz channels are allowed

This adds a helper function to ath/regd.c which can be asked if 4.9GHz channels
are allowed for a given regulatory domain code. This keeps the knowledge of
regdomains and defines like MKK9_MKKC in one place. I'm passing the regdomain
code instead of the ath_regulatory structure because this needs to be called
quite early in the driver inititalization where ath_regulatory is not available
yet in ath5k.

I'm using MKK9_MKKC only because this is the regdomain in the 802.11j enabled
sample cards we got from our vendor. I found some hints in HAL code that this
is used by Atheros to indicate 4.9GHz channels support and that there might be
other domain codes as well, but as I don't have any documentation I'm just
putting in what I need right now. It can be extended later.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>