NetworkPkg: iSCSI should allow to set 6 or 12 length of ISID keyword.
The last 3 bytes of ISID should be able to changed by setting the keyword with
a value with length 6 (only last 3 bytes) or 12 (full ISID) according to the
keyword definition in UEFI configuration namespace website.
Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Fu Siyuan <siyuan.fu@intel.com> Reviewed-by: Ye Ting <ting.ye@intel.com>