ath9k_htc: Fix error handling in add_interface

Addition of a station might fail - handle this error
properly by removing the VAP on the target.
Also, bail out immediately if the max. no of interfaces
has been reached.

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