rt2x00 : modify padding location.

The padding is to be added between header and payload for the only header need
padding case.

Signed-off-by: Benoit PAPILLAULT <benoit.papillault@free.fr>
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>