vdpa/mlx5: Fix dependency on MLX5_CORE

Remove propmt for selecting MLX5_VDPA by the user and modify
MLX5_VDPA_NET to select MLX5_VDPA. Also modify MLX5_VDPA_NET to depend
on mlx5_core.

This fixes an issue where configuration sets 'y' for MLX5_VDPA_NET while
MLX5_CORE is compiled as a module causing link errors.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 1a86b377

 ("vdpa/mlx5: Add VDPA driver for supported mlx5 device")s
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20201007064011.GA50074@mtl-vdi-166.wap.labs.mlnx
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>