ath9k_hw: fix ar9462 selfgen chainmask

When the 9462 is operating in 2G mode and MCI is enabled then
reduce the selfgen chain mask to chain 1. Otherwise poor performance
was reported at short range at Rx side when COEX is enabled.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>