net: dsa: lantiq_gswip: Minor code style improvements

Use one code block when returning because the interface type is
unsupported and also check if some unsupported port gets configured.
In addition fix a double the and use dsa_is_cpu_port() instated of
manually getting the CPU port.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>