nl802154: add missing attribute validation

Add missing attribute validation for several u8 types.

Fixes: 2c21d115 ("net: add NL802154 interface for configuration of 802.15.4 devices")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>