According to UEFI spec, the PXE.Mtftp() should return invalid parameter if the
BufferPtr parameter was NULL and the DontUseBuffer parameter was FALSE.
The DontUseBuffer is only used when perform MTFTP/TFTP read operation.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>