qeth: Fix invalid router settings handling

Give a bad return code when specifying a router setting that is either
invalid or not support on the respective device type. In addition, fall back
the previous setting instead of silently switching back to 'no routing'.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>