rt2500usb: write keys to proper registers

Fix rt2500usb hardware encryption broken by commit
96b61baf
"rt2x00: Clean up USB vendor request buffer functions"

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