net: dsa: bcm_sf2: Unwind errors in correct order

In case we cannot complete bcm_sf2_sw_setup() for any reason, and we
go to the out_unmap label, but the MDIO bus has not been registered yet,
we will hit the BUG condition in drivers/net/phy/mdio_bus.c about the
bus not being registered. Fix this by dedicating a specific lable for
when we fail after the MDIO bus has been successfully registered.

Fixes: 461cd1b0 ("net: dsa: bcm_sf2: Register our slave MDIO bus")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>