Mesh: Fix in Config GATT Proxy Set

According to the 4.3.2.10 Config GATT Proxy Set, message should have only 1
byte.
diff --git a/mesh/config-client.c b/mesh/config-client.c
index 95297bf..edb164b 100644
--- a/mesh/config-client.c
+++ b/mesh/config-client.c
@@ -749,7 +749,7 @@
 static void cmd_proxy_set(int argc, char *argv[])
 {
 	uint16_t n;
-	uint8_t msg[2 + 1 + 4];
+	uint8_t msg[2 + 1];
 	int parm_cnt;
 
 	if (!verify_config_target(target))
@@ -764,7 +764,6 @@
 	}
 
 	msg[n++] = parms[0];
-	msg[n++] = parms[1];
 
 	if (!config_send(msg, n)) {
 		bt_shell_printf("Failed to send \"SET PROXY\"\n");