SourceLevelDebugPkg DebugUsb3: Re-Fix GCC build failures

SourceLevelDebugPkg DebugUsb3: Re-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.

ad6040ec9b5bbc462762331f9738b8e42c0b9c80 needs to be also reverted
when reverting IOMMU support patches, otherwise there will be conflict.
This patch is to re-do ad6040ec9b5bbc462762331f9738b8e42c0b9c80.

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: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>