mdio_bus: fix mdio_register_device when RESET_CONTROLLER is disabled

When CONFIG_RESET_CONTROLLER is disabled, the
devm_reset_control_get_exclusive function returns -ENOTSUPP. This is not
handled in subsequent check and then the mdio device fails to probe.

When CONFIG_RESET_CONTROLLER is enabled, its code checks in OF for reset
device, and since it is not present, returns -ENOENT. -ENOENT is handled.
Add -ENOTSUPP also.

This happened to me when upgrading kernel on Turris Omnia. You either
have to enable CONFIG_RESET_CONTROLLER or use this patch.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Fixes: 71dd6c0d ("net: phy: add support for reset-controller")
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>