mdio_bus: don't use managed reset-controller

Geert Uytterhoeven reported that using devm_reset_controller_get leads
to a WARNING when probing a reset-controlled PHY. This is because the
device devm_reset_controller_get gets supplied is not actually the
one being probed.

Acquire an unmanaged reset-control as well as free the reset_control on
unregister to fix this.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
CC: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David Bauer <mail@david-bauer.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>