tg3: Handle NVRAM absent cases

Some 57780 ASIC revision parts do not have NVRAM.  Code the driver so
that it is tolerant of this configuration.

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