gianfar: Fix stats support

This patch updates the per rx/tx queue stats.
To update the per rx queue stats a new structure has been
introduced rx_q_stats.
The per tx queue stats are updated via the netdev_queue
structure itself.

Note that we update only the tx_packtes, tx_bytes, rx_packets,
rx_bytes and rx_dropped stats on a per queue basis.

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