MdeModulePkg/VarCheckHii: Update data type for variable "ArrayIndex"

MdeModulePkg/VarCheckHii: Update data type for variable "ArrayIndex"

In some case the ArrayIndex with UINT16 may be not large enough to
hold the multiplication result of HiiQuestion->VarOffset * 8;
So this patch update the data type to fix this potential issue.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>