nl802154: add missing attribute validation for dev_type

Add missing attribute type validation for IEEE802154_ATTR_DEV_TYPE
to the netlink policy.

Fixes: 90c049b2 ("ieee802154: interface type to be added")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>