ath9k: remove unnecessary clearing of SC_OP_WAIT_{BEACON,CAB} flags

All SC_OP_WAIT_* flags will be cleared in 'ath9k_conf' when PS mode
is disabled, so we don't have to clear it here.

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