net: macb: Add provision for queue tie off

Currently all RX queues are enabled but a provision to tie off
RX queue may be necessary, for ex., in PM/Wake on LAN conditions.

Disable unused queues by connecting them to a looped descriptor
chain without free slots.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Acked-by: Harini Katakam <harini.katakam@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>