MdePkg UefiLib: Fix wrong DestMax passed to StrCpyS()

MdePkg UefiLib: Fix wrong DestMax passed to StrCpyS()

The second parameter 'DestMax' of StrCpyS() should be the number of
unicode characters, not the size in bytes.

Also, code is modified to keep align with the one in
IntelFrameworkPkg\Library\FrameworkUefiLib\UefiLibPrint.c.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18160 6f19259b-4bc3-4df7-8a09-765794883524