net/mlx5: mitigate Rx doorbell memory barrier

The RQ WQEs must be written in the memory before the HW gets the RQ
doorbell, hence a memory barrier should be triggered after the WQEs
writing and before the doorbell writing.

The current code used rte_wmb barrier which ensures that all the memory
stores were done while it is enough to use rte_cio_wmb barrier for the
local memory stores because the WQEs are in local memory.

CC: stable@dpdk.org
Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>