rt61pci: rt61pci_beacon_update do not free skb twice

The layer above will free the skb in an error case.

Signed-off-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>