net: dsa: return per-port upstream port

The current dsa_upstream_port() helper still assumes a unique CPU port
in the whole switch fabric. This is becoming wrong, as every port in the
fabric has its dedicated CPU port, thus every port has an upstream port.

Add a port argument to the dsa_upstream_port() helper and fetch its CPU
port instead of the deprecated unique fabric CPU port. A CPU or unused
port has no dedicated CPU port, so return itself in this case.

At the same time, change the return value from u8 to unsigned int since
there is no need to limit the size here.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>