rt2x00: Fix padding bug on L2PAD devices.

While reviewing the l2pad function to align both the header and the payload
on a DMA-capable boundary a bug was discovered where the payload would not
be properly aligned. The header_align value was used where the payload_align
value should have been used.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>