cmd: usb_mass_storage: Use NULL for pointer

Use NULL for pointer to fix the following sparse warning:
cmd/usb_mass_storage.c:47:15: warning: Using plain integer as NULL pointer

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>

cmd/usb_mass_storage.c

@@ -44,7 +44,7 @@ static void ums_fini(void)
 	for (i = 0; i < ums_count; i++)
 		free((void *)ums[i].name);
 	free(ums);
-	ums = 0;
+	ums = NULL;
 	ums_count = 0;
 }