net: dsa: bcm_sf2: Do not configure PHYLINK on CPU port

The SF2 binding does not specify that the CPU port should have
properties mandatory for successfully instantiating a PHYLINK object. As
such, there will be missing properties (including fixed-link) and when
attempting to validate and later configure link modes, we will have an
incorrect set of parameters (interface, speed, duplex).

Simply prevent the CPU port from being configured through PHYLINK since
bcm_sf2_imp_setup() takes care of that already.

Fixes: 0e279218 ("net: dsa: Use PHYLINK for the CPU/DSA ports")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>