mdio: mux: make child bus walking more permissive and errors more verbose

If any errors are encountered while walking the device tree structure of
the MDIO bus for children, the code may silently continue, silently
exit, or throw an error and exit.  This make it difficult for device
tree writers to know there is an error.  Also, it makes any error in a
child entry of the MDIO bus be fatal for all entries.  Instead, we
should provide verbose errors describing the error and then attempt to
continue if it all possible.  Also, use of_mdio_parse_addr()

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>