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: 1a86b377aa ("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>

drivers/vdpa/Kconfig

@@ -30,9 +30,7 @@ config IFCVF
 	  be called ifcvf.
 
 config MLX5_VDPA
-	bool "MLX5 VDPA support library for ConnectX devices"
-	depends on MLX5_CORE
-	default n
+	bool
 	help
 	  Support library for Mellanox VDPA drivers. Provides code that is
 	  common for all types of VDPA drivers. The following drivers are planned:
@@ -40,7 +38,8 @@ config MLX5_VDPA
 
 config MLX5_VDPA_NET
 	tristate "vDPA driver for ConnectX devices"
-	depends on MLX5_VDPA
+	select MLX5_VDPA
+	depends on MLX5_CORE
 	default n
 	help
 	  VDPA network driver for ConnectX6 and newer. Provides offloading