net: mdio: Replaced BUG_ON() with WARN()

Killing the kernel because a certain MDIO bus object is not in the
desired state at various points in the registration or unregistration
paths is excessive and is not helping in troubleshooting or fixing
issues. Replace the BUG_ON() with WARN() and print out the MDIO bus name
to facilitate debugging.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>