vdpa/mlx5: Fix tracking of current number of VQs

vdpa/mlx5: Fix tracking of current number of VQs

Modify the code such that ndev->cur_num_vqs better reflects the actual
number of data virtqueues. The value can be accurately realized after
features have been negotiated.

This is to prevent possible failures when modifying the RQT object if
the cur_num_vqs bears invalid value.

No issue was actually encountered but this also makes the code more
readable.

Fixes: c5a5cd3d3217 ("vdpa/mlx5: Support configuring max data virtqueue")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20220111183400.38418-5-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Si-Wei Liu<si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>