ath9k_hw: Fix RX handling for USB devices

Commit "ath9k_hw: Abort rx if hw is not coming out of full sleep in reset"
uncondionally added aborting RX DMA in a HW reset, though it is a bit
unclear as to why this is needed.

Anyway, RX DMA is handled in the target for USB devices, and this would
interfere with normal operations (scanning etc.), so fix this.

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