net: dsa: sja1105: Check for PHY mode mismatches with what PHYLINK reports

PHYLINK being designed with PHYs in mind that can change MII protocol,
for correct operation it is necessary to ensure that the PHY interface
mode stays the same (otherwise clear the supported bit mask, as
required).

Because this is just a hypothetical situation for now, we don't bother
to check whether we could actually support the new PHY interface mode.
Actually we could modify the xMII table, reset the switch and send an
updated static configuration, but adding that would just be dead code.

Cc: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>