NetworkPkg: Abort the PXE process if DHCP has been started by other instance.

PXE need to use extended DHCP options and check received offers in callback
function, so there is no need to continue the PXE process if DHCP driver has
been started by other instance but not PXE driver itself.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>