net/hns3: remove one IO barrier in Rx

net/hns3: remove one IO barrier in Rx

When receiving a packet, hns3 hardware network engine firstly writes the
packet content to the memory pointed by the 'addr' field of the Rx
Buffer Descriptor, secondly fills the result of parsing the packet
include the valid field into the Rx Buffer Descriptor in one write
operation, and thirdly writes the number of the Buffer Descriptor not
processed by the driver to the HNS3_RING_RX_FBDNUM_REG register.

This patch optimizes the Rx performance by removing one rte_io_rmb call
in the '.rx_pkt_burst' ops implementation function named hns3_recv_pkts.
The change as follows:
1. Driver no longer read HNS3_RING_RX_FBDNUM_REG register, so remove one
   rte_io_rmb call, and directly read the valid flag of Rx Buffer
   Descriptor to check whether the BD is ready.
2. Delete the non_vld_descs field from the statistic information of the
   hns3 driver because now it has become a common case that the valid
   flag of Rx Buffer Descriptor read by the driver is invalid.

Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>