net: dsa: mv88e6xxx: Add helper to determining if port has SERDES

Refactor the existing code. This helper will be used for SERDES
statistics.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
1 file changed