MdeModulePkg PcdDxe: ASSERT PcdSetNvStoreDefaultId set

MdeModulePkg PcdDxe: ASSERT PcdSetNvStoreDefaultId set

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1245

In current implementation and according to the description in
MdeModulePkg.dec, PcdSetNvStoreDefaultId should be set in PEI
phase to take effect.

This patch ASSERTs PcdSetNvStoreDefaultId set in PcdDxe to alert
the invalid operation.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>