ath9k: ensure that rx is not enabled during a reset

During a reset, rx buffers are flushed after rx has been disabled. To avoid
race conditions, rx needs to stay disabled during the reset, so avoid any
calls to ath9k_hw_rxena in that case.

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