ath9k: Incorrect band enum causes invalid antenna configuration.

It's a regression from the patch titled "ath9k: Use mac80211's band
macros and remove enum hal_freq_band". MAC80211 band macros can't be
mapped directly with our hal band enum.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>