PcAtChipsetPkg/Rtc: Don't unnecessarily create timezone variable.

PcAtChipsetPkg/Rtc: Don't unnecessarily create timezone variable.

When SetTime() is called with EFI_UNSPECIFIED_TIMEZONE, the code
can optimally not create the private timezone variable because
absence of timezone variable indicates the timezone is unspecified.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19783 6f19259b-4bc3-4df7-8a09-765794883524