NetworkPkg: Fix the issue cannot boot to UEFI Network after reset

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 PXE to start a new round D.O.R.A.
so we need to switch it's policy to static.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18104 6f19259b-4bc3-4df7-8a09-765794883524