bridge: switchdev: Use an helper to clear forward mark

Instead of using ifdef in the C file.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Suggested-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Tested-by: Yotam Gigi <yotamg@mellanox.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>