net: dsa: return after vlan prepare phase

The current code does not return after successfully preparing the VLAN
addition on every ports member of a it. Fix this.

Fixes: 1ca4aa9c

 ("net: dsa: check VLAN capability of every switch")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>