SourceLevelDebugPkg DebugUsb3: Fix GCC build failures

Fix GCC build failures below.
variable 'EvtTrb' set but not used [-Werror=unused-but-set-variable]
variable 'Index' set but not used [-Werror=unused-but-set-variable]

The build failure could only be caught with -D SOURCE_DEBUG_USE_USB3
build flag.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>