MdeModulePkg/NvmExpressPei: Avoid updating the module-level variable

MdeModulePkg/NvmExpressPei: Avoid updating the module-level variable

This commit is out of the scope for BZ-1409. The commit will remove the
call of RegisterForShadow() at the entry point of the driver. By doing so,
the driver is now possible to be executed without being re-loaded into
permanent memory.

Thus, this commit will update the NvmExpressPei driver to avoid updating
the content of a global variable.

Cc: Jian J Wang <jian.j.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>