ath9k_hw: Add hw cap flag for EDMA for the AR9003 family

AR9003 supports extended DMA (EDMA), this comes with some
bells and whistles on top of the legacy DMA that we are used
to. Mark AR9003 and later chips EDMA capable.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>