net: dsa: mv88e6xxx: Prevent suspend to RAM

On one hand, the mv88e6xxx driver has a work queue called in loop
which will attempt register accesses after MDIO bus suspension, that
entirely freezes the platform during suspend.

On the other hand, the DSA core is not ready yet to support suspend to
RAM operation because so far there is no way to recover reliably the
switch configuration.

To avoid the kernel to freeze when suspending with a switch driven by
the mv88e6xxx driver, we choose to prevent the driver suspension and
in the same way, the whole platform.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>