net: dsa: make switch index unsigned

Define the DSA switch index as an unsigned int, because it will never be
less than 0.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -209,7 +209,7 @@ struct dsa_switch {
 	 * Parent switch tree, and switch index.
 	 */
 	struct dsa_switch_tree	*dst;
-	int			index;
+	unsigned int		index;
 
 	/* Listener for switch fabric events */
 	struct notifier_block	nb;