MdePkg: MTftp6: Correct #define value in Mtfp6.h

MdePkg: MTftp6: Correct #define value in Mtfp6.h

defined values of EFI_MTFTP6_ERRORCODE_ILLEGAL_OPERATION and
EFI_MTFTP6_ERRORCODE_FILE_ALREADY_EXISTS are the same. This patch
corrects the value of EFI_MTFTP6_ERRORCODE_ILLEGAL_OPERATION to 4.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hegde Nagaraj P <nagaraj-p.hegde@hpe.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>
Reviewed-By: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>