wlcore: add support for STA CSA with chan contexts

TI wl12xx/wl18xx cards support channel switch via a driver specific
switch_channel op while operating with channel contexts.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>