net: dsa: return directly from dsa_to_port

Return directly from within the loop as soon as the port is found,
otherwise we won't return NULL if the end of the list is reached.

Fixes: b96ddf25 ("net: dsa: use ports list in dsa_to_port")
Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>