mdio_bus: use devm_gpiod_get_optional()

The MDIO reset GPIO is really a classical optional GPIO property case,
so devm_gpiod_get_optional() should have been used, not devm_gpiod_get().
Doing this  saves several LoCs...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>