MdeMoudlePkg/CapsulePei: Optimize AreCapsulesStaged

MdeMoudlePkg/CapsulePei: Optimize AreCapsulesStaged

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1853

AreCapsulesStaged do not need to return the status, only boolean
result is useful. So directly return a boolean value.
Cannot initialize the variable at its definition.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Turner <Michael.Turner@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Zhichao gao <zhichao.gao@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>