mv88e6240 configuration broken for B850v3

Hi Andrew,

I'm currently in the process of updating the GE B850v3 [1] to run a
newer kernel than the one it's currently running. 

This device (and others in the same family) use a mv88e6240 switch to
provide a number of their ethernet ports. The CPU link on the switch is
connected via a PHY, as the network port on the SoM used is exposed via
a PHY.

The ports of the B850v3 stopped working when I upgraded, bisecting
resulted in me finding that this commit was the root cause:

3be98b2d5fbc (refs/bisect/bad) net: dsa: Down cpu/dsa ports phylink
will control

I think this is causing the PHY on the mv88e6240 side of the CPU link
to be forced down in our use case.

I assume an extra check is needed here to stop that in cases like ours,
though I'm not sure what at this point. Any ideas?

Thanks in advance,

Martyn

[1]:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/imx6q-b850v3.dts