rt2x00: Implement HW encryption

Various rt2x00 devices support hardware encryption.

Most of them require the IV/EIV to be generated by mac80211,
but require it to be provided seperately instead of within
the frame itself. This means that rt2x00lib should extract
the data from the frame and place it in the frame descriptor.
During RX the IV/EIV is provided in the descriptor by the
hardware which means that it should be inserted into the
frame by rt2x00lib.

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