rt2x00: Only free skb when beacon_update fails

In rt2x00lib_intf_scheduled_iter() we use the hw->beacon_update()
callback function. This means that it should behave similarly as mac80211
when that uses the function.

This means that the skb should only be freed when beacon_update() has failed,
otherwise the driver is the owner and is responsible for freeing the buffer.

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