MdeModulePkg CapsuleApp: Fix NestedCapsuleHeader->Flags assigned wrong

(FwType == ESRT_FW_TYPE_DEVICEFIRMWARE) ? system : device
should be
(FwType == ESRT_FW_TYPE_SYSTEMFIRMWARE) ? system : device

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ming Shao <ming.shao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>