rt2x00: rt2800: fix zeroing skb structure

We should clear skb->data not skb itself. Bug was introduced by:
commit 0b8004aa12d13ec750d102ba4082a95f0107c649 "rt2x00: Properly
reserve room for descriptors in skbs".

Cc: stable@kernel.org # 2.6.36+
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>