SecurityPkg/TPM2: Update function header of GetDigestFromDigestList()

SecurityPkg/TPM2: Update function header of GetDigestFromDigestList()

Update the return status description and use OUT identifier instead of
IN for Digest parameter, no functionality change.

Cc: Chao B Zhang <chao.b.zhang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>