NetworkPkg: Fix bug in iSCSI mode ipv6 when enabling target DHCP.

NetworkPkg: Fix bug in iSCSI mode ipv6 when enabling target DHCP.

if the server name expressed as a site local address begain with FEC0
when retrieving from dhcpv6 option 59 boot file url, it incorrectly process it
as a dns name.

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