ath5k: Enable 802.11j 4.9GHz frequencies

This enables 4.9GHz frequencies in ath5k if they are allowed as indicated by
the regulatory domain code. Currently this is MKK9_MKKC (0xfe).

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