tg3: Fix NVRAM selftest failures for 5720 devs

This patch fixes NVRAM selftest failures for 5720 devices by fixing the
checksum area size.

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