ath9k: Add routines for switching between active virtual wiphys

ath9k_wiphy_select() can be used to select a virtual wiphy to be
activated. Other virtual wiphys will be paused and once that is done,
the operational channel is changed and the wiphys that are on the
selected channel will be unpaused.

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