Ensure that frames without payload are properly trimmed in
rt2x00queue_insert_l2pad.

This should fix the bug reported by Benoit Papillault in:
http://marc.info/?l=linux-wireless&m=125974773006734&w=2Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>