net: dsa: rtl8366rb: Switch to phylink

This switches the RTL8366RB over to using phylink callbacks
instead of .adjust_link(). This is a pretty template
switchover. All we adjust is the CPU port so that is why
the code only inspects this port.

We enhance by adding proper error messages, also disabling
the CPU port on the way down and moving dev_info() to
dev_dbg().

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>