NetworkPkg: reset DHCP child when leaving PXE LoadFile.

NetworkPkg: reset DHCP child when leaving PXE LoadFile.

The DHCP4 can have only one configured child instance so we need to reset
the DHCP4 child when leaving PXE driver's LoadFile() function, otherwise the
other programs which also need to use DHCP4 (like HTTP boot) will be impacted.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18616 6f19259b-4bc3-4df7-8a09-765794883524