Fix SourceLevelDebugPkg build failure when the debug port is USB.

Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14222 6f19259b-4bc3-4df7-8a09-765794883524