FmpDevicePkg FmpDxe: Use local variable to store test key digest size

FmpDevicePkg FmpDxe: Use local variable to store test key digest size

Some static tool reports "the condition in 'if' statement is constant".

This patch updates the code to use local variable to store test key
digest size. 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>