rt2x00: Centralize RX packet alignment handling in rt2x00lib.

When rt2x00pci will be switched over to dynamically mapped skb's
instead of statically allocated DMA buffers, it no longer can handle
alignment of RX packets in a copy step, and needs to implement the
same scheme as rt2x00usb does.

In order to make the patch on dynamically mapped skb's smaller,
already centralize the alignment handling into rt2x00lib. This allows
us to move more code in rt2x00lib, and thus remove code duplication
between rt2x00usb and rt2x00pci.

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