net: dsa: mv88e6xxx: Cache the port cmode

The ports CMODE indicates the type of link between the MAC and the
PHY. It is used often in the SERDES code. Rather than read it each
time, cache its value.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>