ath9k: Make start/stop operations aware of virtual wiphys

Instead of always going through initialization/deinitialization steps,
do this only for the first/last wiphy to not break the other wiphys.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>