MdeModulePkg RamDiskDxe: Fix wrong HII behavior for more than 8 RAM disks

MdeModulePkg RamDiskDxe: Fix wrong HII behavior for more than 8 RAM disks

The RamDiskDxe driver originally uses a variable-length HII varstore to
retrieve the HII checkbox status of each registered RAM disk.

However, HII does not support the variable-length varstore feature.
Therefore, only the checkbox status for the first 8 RAM disks are tracked
for the following definition of HII varstore structure considering the
alignment:

typedef struct {
  UINT64    Size;
  UINT8     RamDiskList[0];
  } RAM_DISK_CONFIGURATION;

This commit uses the private data of each registered RAM disks to track
the HII checkbox status instead to resolve the issue.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>