net: ethernet: ti: cpsw: unsync mcast entries while switch promisc mode

After flushing all mcast entries from the table, the ones contained in
mc list of ndev are not restored when promisc mode is toggled off,
because they are considered as synched with ALE, thus, in order to
restore them after promisc mode - reset syncing info. This fix
touches only switch mode devices, including single port boards
like Beagle Bone.

Fixes: commit 5da19489
("net: ethernet: ti: cpsw: fix lost of mcast packets while rx_mode update")

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>