ath5k: don't enable probe request rx for STAs

AR5K_RX_FILTER_PROBEREQ enables reception of probe requests,
but the filter flag FIF_BCN_PRBRESP_PROMISC is actually about
receiving beacons and probe _responses_, so we shouldn't
turn on the filter when scanning.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>