net: Fix driver name for mdio-gpio.c

Since commit
"7488876... dt/net: Eliminate users of of_platform_{,un}register_driver"
there are two platform drivers named "mdio-gpio" registered.
I renamed the of variant to "mdio-ofgpio".

Signed-off-by: Dirk Eibach <eibach@gdsys.de>
Signed-off-by: David S. Miller <davem@davemloft.net>