NetworkPkg/IScsiDxe: Switch IP4 configuration policy to Static before DHCP

NetworkPkg/IScsiDxe: Switch IP4 configuration policy to Static before DHCP

DHCP4 service allows only one of its children to be configured in the active
state. If the DHCP4 D.O.R.A started by IP4 auto configuration and has not
been completed, the Dhcp4 state machine will not be in the right state for
the iSCSI to start a new round D.O.R.A. So, we need to switch it's policy to
static.

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