FmpDevicePkg FmpDxe: Initialize DeviceLibLowestSupportedVersion

Some static tool reports "DeviceLibLowestSupportedVersion" is used,
but is uninitialized. It is false positive reporting based because
DeviceLibLowestSupportedVersion will have assigned value after
FmpDeviceGetLowestSupportedVersion() returns SUCCESS.

This patch updates the code to initialize
DeviceLibLowestSupportedVersion
to DEFAULT_LOWESTSUPPORTEDVERSION before calling
FmpDeviceGetLowestSupportedVersion().
It can pass the static tool's check.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>