Make link mode case independent

The link mode is printed in upper case, and following the general
rule that ip command output should work on input, allow either case.

ip/iplink.c

@@ -134,9 +134,9 @@ struct link_util *get_link_kind(const char *id)
 
 int get_link_mode(const char *mode)
 {
-	if (strcmp(mode, "default") == 0)
+	if (strcasecmp(mode, "default") == 0)
 		return IF_LINK_MODE_DEFAULT;
-	if (strcmp(mode, "dormant") == 0)
+	if (strcasecmp(mode, "dormant") == 0)
 		return IF_LINK_MODE_DORMANT;
 	return -1;
 }