ath9k: fix double calls to ath_radio_enable

With the enable_radio being uninitialized, ath_radio_enable() might be
called twice, which can leave some hardware in an undefined state.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>