net: stmmac: fix reset gpio free missing

Commit 984203ce ("net: stmmac: mdio: remove reset gpio free")
removed the reset gpio free, when the driver is unbinded or rmmod,
we miss the gpio free.

This patch uses managed API to request the reset gpio, so that the
gpio could be freed properly.

Fixes: 984203ce ("net: stmmac: mdio: remove reset gpio free")
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: David S. Miller <davem@davemloft.net>