ath5k: Put hardware in PROMISC mode if there is more than 1 stations.

It seems ath5k has issues receiving broadcast packets (ARPs) when
using multiple STA interfaces associated with multiple APs.
This patch ensures the NIC is always in PROMISC mode if there
are more than 1 stations associated.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>