NetworkPkg/IScsiDxe: Fix the ISCSI connection failure in certain case.

NetworkPkg/IScsiDxe: Fix the ISCSI connection failure in certain case.

The ISCSI connection will fail for the first time if the target info is
retrieved from DHCP and expressed as URI format. The issue is caused by
the missing DNS protocol dependency check during the driver support
function.

This patch is to fix the above issue.

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