Mellanox: Add new mlxsw_spectrum firmware 13.1703.4

This new firmware contains a fix for MC-aware mode.
Signed-off-by: Petr Machata <petrm@mellanox.com>

WHENCE

@@ -4086,6 +4086,7 @@ File: mellanox/mlxsw_spectrum-13.1420.122.mfa2
 File: mellanox/mlxsw_spectrum-13.1530.152.mfa2
 File: mellanox/mlxsw_spectrum-13.1620.192.mfa2
 File: mellanox/mlxsw_spectrum-13.1702.6.mfa2
+File: mellanox/mlxsw_spectrum-13.1703.4.mfa2
 
 Licence:
  Copyright (c) 2017-2018 Mellanox Technologies, Ltd. All rights reserved.