MdeModulePkg BaseSerialPortLib: Fix VS2010 build error

MdeModulePkg BaseSerialPortLib: Fix VS2010 build error

When overriding compiler options '/GL' with '/GL-', VS2010 will report
warning C4701 potentially uninitialized local variable for 'LcrParity'
and 'LcrStop' in function SerialPortSetAttributes().

This commit fixes this build issue.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19628 6f19259b-4bc3-4df7-8a09-765794883524