rt2x00: Reorganize L2 padding inserting function.

Simplify the rt2x00queue_insert_l2pad function by handling the alignment
operations one by one. Do not special case special circumstances.
Basically first perform header alignment, and then perform payload alignment
(if any payload does exist). This results in a properly aligned skb.

The end result is better readable code, with better results, as now L2 padding
is inserted only when a payload is actually present in the frame.

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