net: dsa: loop: Initialize err in dsa_loop_vlan_dump

Dan's static checker reported the following:

	drivers/net/dsa/dsa_loop.c:223 dsa_loop_port_vlan_dump()
	error: uninitialized symbol 'err'.

which could happen if we do hit the continue statement for each iteration of
the loop. Initialize err to 0 here.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 98cd1552 ("net: dsa: Mock-up driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>