This patch fix an issue that happen when changing the MAC address when
the port is down, described as follows:
1. Set the port down.
2. Change the MAC address - mlx4_en_set_mac() will change dev->dev_addr.
3. Set the port up - will result in mlx4_en_do_uc_filter that will
   remove the prev_mac entry from the mac_hash db.
4. Changing the MAC address again will eventually trigger the call to
   mlx4_en_replace_mac() in order to replace prev_mac with dev_addr but
   the prev_mac entry is already not exist in the mac_hash db therefore
   the operation fails.

The fix is to set the prev_mac with the new MAC address so in step 3
above, after setting the port up mlx4_en_get_qp() is updating the
mac_hash with the entry of dev_addr which is equal to prev_mac.
Therefore in step 4, when calling mlx4_en_replace_mac, the entry related
to prev_mac exist in mac_hash and the replace operation succeed.
Reviewed-by: Eyal Perry <eyalpe@mellanox.com>
Signed-off-by: Shani Michaeli <shanim@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>