net: dsa: propagate error code from dsa_slave_phy_setup

In case we cannot attach to our slave netdevice PHY, error out and
propagate that error up to the caller: dsa_slave_create().

Fixes: 0d8bcdd3 ("net: dsa: allow for more complex PHY setups")
Signed-off-by: Andrey Volkov <andrey.volkov@nexvision.fr>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>