Enhance TargetName check when save iSCSI configuration.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ye Ting  <ting.ye@intel.com>
Reviewed-by: Wu, Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu, Siyuan <siyuan.fu@intel.com>


git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@16167 6f19259b-4bc3-4df7-8a09-765794883524