ath9k_hw: Fix invalid MCI GPM index access/caching

There is a possibility that AR_MCI_GPM_1 register can
return 0xdeadbeef and this results in caching of invalid
GPM index in ar9003_mci_is_gpm_valid. Ensure we
have appropriate checks to avoid this.

Cc: xijin luo <xijin@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>