ath9k: Fix rx of mcast/bcast frames in PS mode with auto sleep

The functionality to keep the device awake until it is done with
the rx of any mcast/bcast frames which are pending on AP should
also be added to the hardwares which support auto sleep feature.
This patch fixes frequent failures in ARP resolution when it is
initiated by the other end. Currently auto sleep is enabled only
for ar9003 in ath9k.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>