net/ucc_geth: Assign six threads to Rx for UEC

In the case the QE has 46 SNUMs for the threads to support four UCC
Ethernet at 1000Base-T simultaneously.

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>