net: dsa: loop: Print when registration is successful

We have a number of error conditions that can lead to the driver not
probing successfully, move the print when we are sure
dsa_register_switch() has suceeded. This avoids repeated prints in case
of probe deferral for instance.

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