net: lantiq: Fix returned value in case of error in 'xrx200_probe()'

Return 'err' in the error handling path instead of 0.
Return explicitly 0 in the normal path, instead of 'err', which is known
to be 0 at this point.

Fixes: fe1a5642 ("net: lantiq: Add Lantiq / Intel VRX200 Ethernet driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>