rt2x00: Reorganize padding & L2 padding

The old function rt2x00queue_payload_align() handled
both adding and removing L2 padding and some basic
frame alignment. The entire function was being abused
because it had multiple functions and the header length
argument was somtimes used to align the header instead
of the payload.

Additionally there was a bug when inserting L2 padding
that only the payload was aligned but not the header. This
happens when the header wasn't aligned properly by mac80211,
but rt2x00lib only moves the payload.

A secondary problem was that when removing L2 padding during
TXdone or RX the skb wasn't resized to the proper size.

Split the function into seperate functions each handling
its task as it should.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>