net: phy: mxl-gpy: disable interrupts on GPY215 by default

The interrupts on the GPY215B and GPY215C are broken and the only viable
fix is to disable them altogether. There is still the possibilty to
opt-in via the device tree.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>