Did proper error handling when SetVariable failed, and put RTC write operation at the behind of SetVariable, if SetVariable failed, RTC content could not be changed.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Elvin Li <elvin.li@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/PcAtChipsetPkg@15338 6f19259b-4bc3-4df7-8a09-765794883524