vdpa/mlx5: fix completion queue initialization

Vdpa device failed to initialize 2nd VQ during setup. From FW syndrome,
unsupported CQE size was specified in CQ initialization attributes.

The unsupported CQE size comes from uninitialized stack struct data, and
the struct has new fields defined recently which are not initialized in
vdpa code.

This patch initializes cq creation attributes with zero to avoid such
random data.

Fixes: 79a7e409a2f6 ("common/mlx5: prepare support of packet pacing")

Signed-off-by: Xueming Li <xuemingl@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>