at76c50x-usb: Move function at76_join() several lines up

This patch does a simple code move of at76_join() so that
at76_mac80211_tx() follows at76_join() in the driver's source file.

This is a preparatory patch for the following patch where we need
to call at76_join() from at76_mac80211_tx() in order to
authenticate successfully with a bssid.

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