forcedeth: run loopback test only on chipsets that support it

The driver incorrectly run loopback test on chips that don't support it.
Loopback test is only supported by chips that has DEV_HAS_TEST_EXTENDED
flag and returns 4 (NV_TEST_COUNT_EXTENDED) as test count.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>