ath9k_htc: Fix packet injection

To inject a packet in monitor mode, a dummy station has
to be associated with the monitor interface in the target.
Failing to do this would result in a firmware crash on the device.

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