tg3: Fix NVRAM selftest

The tg3 NVRAM selftest actually fails when validating the checksum of
the legacy NVRAM format.  However, the test still reported success
because the last update of the return code was a success from the NVRAM
reads.  This patch fixes the code so that the error return code defaults
to a failure status.  Then the patch fixes the reason why the checsum
validation failed.

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