rt2x00: Align RX descriptor to 4 bytes

Some architectures give problems when reading
RX frame descriptor words when the descriptor
is not aligned on a 4 byte boundrary.

Due to optimalizations for the ieee80211 payload
4 byte alignment, it is no longer guarenteed
that the descriptor is placed on the 4 byte
boundrary (In fact, for rt73usb it is absolutely
never aligned to 4 bytes, for rt2500usb it depends
on the length of the payload).

This will copy the descriptor to a 4 byte aligned
location before it is read for the first time.
This will also move the payload data alignment
in rt2x00usb (instead of inside the driver) where
it has always belonged.

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