MdeModulePkg: Refine codes of iSCSI driver

The RSDT is only used when the bios need to support ACPI 1.0
version. When change PcdAcpiExposedTableVersions to 0x3C, it
will not support ACPI 1.0. The default is 0x3E.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by:  Eric Dong <eric.dong@intel.com>