ath9k_htc: Unify target capability updating

Update capabilites on the target once, when
start() is called, there is no need for redundant
updating on adding an interface.

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