net: dsa: sja1105: unregister the MDIO buses during teardown

The call to sja1105_mdiobus_unregister is present in the error path but
absent from the main driver unbind path.

Fixes: 5a8f0974 ("net: dsa: sja1105: register the MDIO buses for 100base-T1 and 100base-TX")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>