[netdevice] Reset network device index when last device is unregistered

When functioning as an EFI driver, drivers can be disconnected and
reconnected multiple times (e.g. via the EFI shell "connect" command,
or by running an executable such as ipxe.efi which will temporarily
disconnect existing drivers).

Minimise surprise by resetting the network device index to zero
whenever the last device is unregistered.  This is not foolproof, but
it does handle the common case of having all devices unregistered and
then reregistered in the original order.

Signed-off-by: Michael Brown <mcb30@ipxe.org>