net: mdio: mux-mmioreg: Simplified with scoped function

Avoids the need for manual cleanup of_node_put() in early exits
from the loop by using for_each_available_child_of_node_scoped().

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>