ethdev: fix xstat name of basic stats per queue

ethdev: fix xstat name of basic stats per queue

As described in doc/guides/prog_guide/poll_mode_drv.rst,
the naming scheme for the xstats is parts separated with underscore:
	* direction
	* detail 1
	* detail 2
	* detail n
	* unit
where detail 1 can be "q" followed with a queue number.
It means the name of the stats per queue should be rx_qN_* or tx_qN_*.

The second underscore was missing so far.
Fixing the basic xstat names may be considered an API change,
that's why it should not be backported.

While fixing this mistake, some examples of the naming scheme
are given as part of the API documentation of rte_eth_xstat_name.
More proposals about standardizing statistics:
	http://fast.dpdk.org/events/slides/DPDK-2019-09-Ethernet_Statistics.pdf

Fixes: bd6aa172cf35 ("ethdev: fetch extended statistics with integer ids")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Ciara Power <ciara.power@intel.com>