Fix an issue that execute "map -r" in UEFI shell cannot reinstate iSCSI session with Microsoft iSCSI target after unplug/plug network cable.

Signed-off-by: Ye Ting  <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@14782 6f19259b-4bc3-4df7-8a09-765794883524