net: dsa: mv88e6xxx: return NULL when no PCS is present

Rather than returning an EOPNOTSUPP error pointer when the switch
has no support for PCS, return NULL to indicate that no PCS is
required.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>