drivers: net: cpsw: dual_emac: fix reducing of rx descriptor during ifdown

In Dual EMAC, when both interface are up and while doing ifdown with heavy
traffic then skbs already processed by DMA from that slave emac has to be
requeued as still the other interface is up and running.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>