NetworkPkg: Fix incorrect parameter check in PXE.Mtftp() function.

NetworkPkg: Fix incorrect parameter check in PXE.Mtftp() function.

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>