mac80211: iterate the virtual monitor interface

If the virtual monitor interface is requested
by the driver, it should also be iterated over
when the driver wants to iterate all active
interfaces.

To allow that protect it with the iflist_mtx.

Change-Id: I58ac5de2f4ce93d12c5a98ecd2859f60158d5d69
Signed-off-by: Johannes Berg <johannes.berg@intel.com>