mac80211: allow no mac address until firmware load

Originally by Johannes Berg. This patch adds support for devices that do not
report their MAC address until the firmware is loaded. While the address is not
known, a multicast on is used.

Signed-off-by: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>