rdma: always use 64 byte CQEs for MLX5

rdma: always use 64 byte CQEs for MLX5

When DPDK MLX PMDs are built, and the DPDK plugin is loaded, DPDK may
set the MLX5_CQE_SIZE environment variable to 128. This causes the RDMA
plugin to be unable to create completion queues. Since the RDMA plugin
expects the CQEs to be 64 bytes, set the cqe_size explicitly when
creating the CQ. This avoids any issues with different values for the
MLX5_CQE_SIZE environment variable.

Type: improvement
Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: Idfd078d3045a4dcb674325ef36f85a89df6fbebc