mac80211: move bss_conf into vif

Move bss_conf into the vif struct so that drivers can
access it during ->tx without having to store it in
the private data or similar. No driver updates because
this is only for when they want to start using it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>