MdePkg/UefiDebugLibStdErr: Pass the correct buffer size

MdePkg/UefiDebugLibStdErr: Pass the correct buffer size

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2302

The second argument of "UnicodeVSPrintAsciiFormat" is "BufferSize",
which takes the size of the buffer in bytes. Replace the currently
used MAX_DEBUG_MESSAGE_LENGTH usage, which is the buffer's length,
with the actual buffer size.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Marvin Haeuser <mhaeuser@outlook.de>
Reviewed-by: Liming Gao <liming.gao@intel.com>