MdeModulePkg: Fix bug in DxeHttplib when converting port number.

MdeModulePkg: Fix bug in DxeHttplib when converting port number.

Http boot on X64 platform is faild, this is caused by the incorrect
type conversion when getting port number from Url.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>