mac80211: Add RTNL version of ieee80211_iterate_active_interfaces

Since commit e38bad4766a110b61fa6038f10be16ced8c6cc38
	mac80211: make ieee80211_iterate_active_interfaces not need rtnl
rt2500usb and rt73usb broke down due to attempting register access
in atomic context (which is not possible for USB hardware).

This patch restores ieee80211_iterate_active_interfaces() to use RTNL lock,
and provides the non-RTNL version under a new name:
	ieee80211_iterate_active_interfaces_atomic()

So far only rt2x00 uses ieee80211_iterate_active_interfaces(), and those
drivers require the RTNL version of ieee80211_iterate_active_interfaces().
Since they already call that function directly, this patch will automatically
fix the USB rt2x00 drivers.

v2: Rename ieee80211_iterate_active_interfaces_rtnl

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>