ath9k_htc: Allow upto two simultaneous interfaces

Multiple interfaces can be configured if a slot is free
on the target. Monitor mode also requires a slot.

The maximum number of stations that can be handled in
the firmware is 8, manage the station slots accordingly.

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