ath: Add a driver_info bitmask field

The driver_info stores the device category information which
is used to load appropriate device firmware, select firmware offset
and eeprom starting location. The driver_info is accessed across
ath9k_htc and ath9k_hw. Hence placed under common structure.

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