can: gw: Use nla_policy to validate netlink attributes

Also use nla_get_u32() instead of nla_memcpy() to access u32 attribtues.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>