ath9k: fix race in setting ATH_OP_INVALID

The commit "ath9k: move sc_flags to ath_common" moved setting
ATH_OP_INVALID flag below ieee80211_register_hw. This is causing
the flag never being cleared randomly as the drv_start is called
prior to setting flag. Fix this by setting the flag prior to
register_hw.

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