can: gw: Remove pointless casts

No need to cast return value of nla_data()

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>