gianfar: Add per queue structure support

This patch introduces per tx and per rx queue structures.
Earlier the members of these structures were inside the
gfar_private structure.

Moving forward if we want to support multiple queues, we need
to refactor the gfar_private structure so that introduction of
multiple queues is easier.

Signed-off-by: Sandeep Gopalpet <Sandeep.Kumar@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>