[undi] Clean up driver and device name information

[undi] Clean up driver and device name information

Fix the <NULL> driver name reported by "ifstat" when using the undipci
driver (due to the unnecessary extra device node inserted as a child
of the PCI device).

Remove the "UNDI-" prefix from device names since the driver name is
also now visible via "ifstat", and tidy up the device name to match
the format used by standard PCI devices.

The output from "ifstat" now resembles:

  iPXE> ifstat
  net0: 52:54:00:12:34:56 using undipci on 0000:00:03.0

  iPXE> ifstat
  net0: 52:54:00:12:34:56 using undionly on 0000:00:03.0

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