ath9k: process rx packet if we are waiting for CAB

If we are in PS mode, we have to process the received frame if
the SC_OP_WAIT_FOR_CAB bit is set.

Changes-licensed-under: ISC
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>