dsa: make dsa_switch_setup check for valid port names

This patch changes dsa_switch_setup() to ensure that at least one valid
valid port name is specified and will bail out with an error in case we
walked the maximum number of port with a valid port name found.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>