Add the missing parameter comments for BaseSerialPortLib16550 lib. MdePkg: Fix Clang build failure

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gao, Liming <liming.gao@intel.com>


git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@15796 6f19259b-4bc3-4df7-8a09-765794883524