net: dsa: Pass a port to get_tag_protocol()

A number of drivers want to check whether the configured CPU port is a
possible configuration for enabling tagging, pass down the CPU port
number so they verify that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>