ath9k: Apply CUS227 specific TX gain values

CUS227, which is an AR9340 based card used in Qualcomm's
Allplay platforms requires a custom TX gain array, based
on the index 7. Add suport for this.

Cc: Michael Larson <mlarson@qce.qualcomm.com>
Cc: Stephen Collmeyer <scollmey@qce.qualcomm.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>