MdeModulePkg: Fix the wrong IpSb->State update

MdeModulePkg: Fix the wrong IpSb->State update

This patch is used to fix the wrong IpSb->State update issue.

Issue reproduce steps:
1 .First PXE boot, then boot to shell;
2. ifconfig -s eth0 dhcp (Success);
3. Reboot and do PXE, then boot to shell;
4. ifconfig -s eth0 dhcp (Platform failed to get IP address no matter
   how many times retried.)

Root cause:
On step3 reboot, policy is DHCP (Changed by step2). So, Ip4Dxe driver
will try to get one IP address from DHCP server automatically. Before
it get the IP address successfully, the IpSb->State will be always in
IP4_SERVICE_STARTED status until the Instance->Dhcp4Event is triggered,
then it can be changed to IP4_SERVICE_CONFIGED. But the DHCP process
will be interrupted by PXE boot, which will change the policy to static,
and the Instance->Dhcp4Event will be also closed directly. However,
current implementation doesn't update the IpSb->State to
IP4_SERVICE_UNSTARTED status in such case. So, failure happened.

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