rt2x00: Rework alignment check.

Rework the alignment check in rt2x00dev.c to be independent of any
potential alignment measures that may be taken before. Just check
whether the payload is aligned based on the pointer addresses.

Note: This is preparatory for the dynamically mapped skb buffers
for the PCI drivers, as these need 4-byte alignment instead of the
currently enforced offset by 2 bytes.

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>