Sync patch r15338 from main trunk.
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: Jeff Fan <jeff.fan@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/branches/UDK2010.SR1/PcAtChipsetPkg@15348 6f19259b-4bc3-4df7-8a09-765794883524