net/hns3: fix Rx buffer size

net/hns3: fix Rx buffer size

Currently, rx_buf_size of hns3 PMD driver is fixed on, and it's value
depends on the firmware which will decrease the flexibility of PMD.

The receive side mbufs was allocated from the mempool given by upper
application calling rte_eth_rx_queue_setup API function. So the memory
chunk used for net device DMA is depend on the data room size of the
objects in this mempool. Hns3 PMD driver should set the rx_buf_len
smaller than the data room size of mempool and our hardware only support
the following four specifications: 512, 1024, 2148 and 4096.

Fixes: bba636698316 ("net/hns3: support Rx/Tx and related operations")
Cc: stable@dpdk.org
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>