[undi] Ensure only one UNDI instance is loaded

[undi] Ensure only one UNDI instance is loaded

Loading multiple UNDI instances would be useful in systems that have
several network cards with vendor PXE ROMs.  However, we cannot rely on
UNDI ROMs working correctly with multiple instances loaded
simultaneously.

The gPXE UNDI driver supports the following multi-NIC configurations:
1. Chainloading undionly.kpxe on a specific NIC.
2. Loading the UNDI driver for the first probed device and ignoring all
   other UNDI devices in the system.

This patch refuses to probe additional UNDI devices so there can never
be multiple instances of UNDI loaded.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Marty Connor <mdc@etherboot.org>