hwmon: (max6621) Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/hwmon/max6621.c

@@ -156,7 +156,7 @@ max6621_is_visible(const void *data, enum hwmon_sensor_types type, u32 attr,
 		default:
 			break;
 		}
-
+		break;
 	default:
 		break;
 	}