SignedCapsulePkg SystemFirmwareUpdateDxe: Fix failure caused by d69d922

d69d9227d046211265de1fab5580c50a65944614 caused system firmware update
failure. It is because FindMatchingFmpHandles() is expected to return
handles matched, but the function returns all handles found.

This patch is to fix the issue.
This patch also assigns mSystemFmpPrivate->Handle for "case 1:" path
in case the Handle is needed by other place in future.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jiewen Yao <jiewen.yao@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>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>