ath9k_htc: Handle host RX disable

The MIB counters used by ANI have to be disabled
on the host because the FW doesn't do it on the target side.
Also, flush the receive buffers before initializing
RX on the target.

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