rt2x00 : trim the skb after having the l2pad removed.

Otherwise we end up truncating the skb before removing the l2pad
thus we might have the truncated part become garbage while getting
it back in remove_l2pad.
For the same issue: remove the skb_trim from the rt2800 fill_rxdone
(it is done after l2pad removal in rt2x00lib_rxdone).

Signed-off-by: Alban Browaeys <prahal@yahoo.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>