ath9k_hw: skip asynch fifo enablement to AR9003

The asynch fifo code is specific to >= AR9287 so stuff it
into the AR9002 hardware family code and skip it for AR9003
cards.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>