ath9k: wake up the chip for TSF reset

If we are in NETWORK SLEEP state, AR_SLP32_TSF_WRITE_STATUS limit
always exceeds in 'ath9k_hw_reset_tsf', because reading of the
AR_SLP3 register always return with the magic 0xdeadbeef value.

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