net: cpsw: fix cpdma rx descriptor leak on down interface

This patch fixes a CPDMA RX Descriptor leak that occurs after taking
the interface down when the CPSW is in Dual MAC mode. Previously
the CPSW_ALE port was left open up which causes packets to be received
and processed by the RX interrupt handler and were passed to the
non active network interface where they were ignored.

The fix is for the slave_stop function of the selected interface
to disable the respective CPSW_ALE Port from forwarding packets. This
blocks traffic from being received on the inactive interface.

Signed-off-by: Schuyler Patton <spatton@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>