phy dp83867: Fix compilation with CONFIG_OF_MDIO=m

When CONFIG_OF_MDIO is configured as module, the #define for it really
is CONFIG_OF_MDIO_MODULE, not CONFIG_OF_MDIO. So if we are compiling it
as module, the dp83867 doesn't see that OF_MDIO was selected and doesn't
read the dt rgmii parameters.

The fix is simple: Use IS_ENABLED(). It checks for both - module as well
as compiled in code.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>