ath9k: remove ath_rx_ps_back_to_sleep helper

This helper only clears the SC_OP_WAIT_FOR_{BEACON,CAB} flags.
Remove it and clear these flags directly in the approptiate
places instead.

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