net: mdiobus: avoid BUG_ON() in mdiobus accessors

Avoid using BUG_ON() in the mdiobus accessors, prefering instead to use
WARN_ON_ONCE() and returning an error.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>