dsa: fix warning in net/dsa/mv88e6xxx.c

this warning:

  net/dsa/mv88e6xxx.c: In function ‘mv88e6xxx_poll_link’:
  net/dsa/mv88e6xxx.c:361: warning: ‘port_status’ may be used uninitialized in this function

triggers because GCC does not recognize the (correct) error flow
between 'link' and 'port_status'.

Annotate it.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/mv88e6xxx.c

@@ -358,7 +358,7 @@ void mv88e6xxx_poll_link(struct dsa_switch *ds)
 
 	for (i = 0; i < DSA_MAX_PORTS; i++) {
 		struct net_device *dev;
-		int port_status;
+		int uninitialized_var(port_status);
 		int link;
 		int speed;
 		int duplex;