rt2x00: Set SKBDESC_L2_PADDED in RX path

When the RX descriptor indicates the frame was L2 padded,
the SKBDESC_L2_PADDED flag should be set to make sure the
L2 padding is removed before it is send to mac80211.

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