ieee802154/mrf24j40: make sure we do not override return values

If we run into an error during rx we set the the error code in ret, but override
it afterwards. Using a different variable for the extra case avoids this
situation.

CID: 1226982, 1226983
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>
Cc: Alan Ott <alan@signal11.us>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>