net: dsa: lantiq_gswip: drop bogus drvdata check

The platform-device driver data is set on successful probe and will
never be NULL on remove (or we have much bigger problems).

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>