net: dsa: b53: do not enable EEE on bcm63xx

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: b53: do not enable EEE on bcm63xx

BCM63xx internal switches do not support EEE, but provide multiple RGMII ports where external PHYs may be connected. If one of these PHYs are EEE capable, we may try to enable EEE for the MACs, which then hangs the system on access of the (non-existent) EEE registers.

Fix this by checking if the switch actually supports EEE before attempting to configure it.