Staging: at76_usb: update drivers/staging/at76_usb w/ mac80211 port

This updates drivers/staging/at76_usb to correspond with the latest
version previously available in wireless-testing.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>