ieee802154: cc2520: check for return values in cc2520_filter()

neither ram nor register write return values have been checked here.
Checking both now. Assign ret with 0 as all other assignments are inside
if blocks and might not happen before we return ret.

CID: 1230469
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>