tg3: Fix napi assignments in loopback test

The 57765 lacks TSS support.  This renders the napi assignments
incorrect in the loopback test function.  This patch fixes the problem.

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