MdeModulePkg/DxeHttpLib: Correct the return status for the HTTP Port/ContentLength

MdeModulePkg/DxeHttpLib: Correct the return status for the HTTP Port/ContentLength

Replace AsciiStrDecimalToUintn with AsciiStrDecimalToUintnS to
return the correct status for the HTTP Port/ContentLength.

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