net: dsa: mv88e6xxx: Fix W=1 warning with !CONFIG_OF

When building on platforms without device tree, e.g. amd64, W=1 gives
a warning about mv88e6xxx_mdio_external_match being unused. Replace
of_match_node() with of_device_is_compatible() to prevent this
warning.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>