ath9k_hw: add an extra delay when reseting AR_RTC_RESET

Without this we could start trying to work with the device without
it being fully functional yet and loose some packets upon resume.

Cc: Aeolus Yang <aeolus.yang@atheros.com>
Cc: Madhan Jaganathan <madhan.jaganathan@atheros.com>
signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>