mac80211: Introduce a generic commit() to apply changes

This patch introduces a generic commit() function which initiate a
new network joining process. It should be called after some interface
config changes, so that the changes get applied more cleanly. Currently
set_ssid() and set_bssid() call it. Others can be added in future
patches.

In version 1 the header files was forgotten, sorry.

Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>