MdeMoudlePkg/CapsulePei: Optimize GetScatterGatherHeadEntries

MdeMoudlePkg/CapsulePei: Optimize GetScatterGatherHeadEntries

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

Rename the MACRO from MAX_SG_LIST_HEADS to DEFAULT_SG_LIST_HEADS.
GetScatterGatherHeadEntries: use allocated buffer instead of fixed
array to handle the condition which the SG list is larger then the
array size.

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>