ath9k_htc: Handle monitor mode properly for HTC devices

No need to inform about monitor interface changes to
firmware. Set the HW mode to monitor type based on mac80211
indication flag is sufficient.

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