at76c50x-usb: Extract bssid from authentication frame

The driver at76c50x-usb is unable to authenticate with an AP since
kernel 2.6.31 for the following reason: The join command of the firmware
needs to be sent with the right bssid before any transmission can start.
Before kernel 2.6.31 mac80211 informed its drivers about the changing
bssid early enough for at76c50x-usb but during the development of 2.6.31
mac80211's behaviour changed. Now a new bssid is set after the
association.

This patch changes the tx routine of the driver at76c50x-usb in such a
way that a new bssid is extracted from an authentication frame and the
join command with that bssid is processed.

Signed-off-by: Sebastian Smolorz <sesmo@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>