tg3: Nullify RSS for loopback test

The loopback test assumes all traffic goes to the first rx queue.  There
is a 1 in 4 chance this won't be true if RSS is enabled though.  This
patch reprograms the RSS indirection table to route all rx packets to
the first queue.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>