Refactor logic for gathering statistics into a per-queue function.
This improves readability of the driver statistics' flows.

In addition, this would be required by the XDP forwarding queues
[as we'll need the Txq statistics gathering methods for those as well].
Signed-off-by: Yuval Mintz <Yuval.Mintz@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>