mlx4: decreasing ref count when removing mac

For older FW versions, when a Mac address removed from Mac table,
we should set 0 for reference count for the corresponding Mac index.
Fixes a bug where removing Mac from the table still left that entry as
invalid.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Tested-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: David S. Miller <davem@davemloft.net>