natsemi: Update locking documentation

The documentation regarding synchronisation at the head of the natsemi
driver was badly bitrotted so replace it with a general statement about
the techniques used which is less likely to bitrot.

Also remove the note saying these chips are uncommon - it makes little
difference but they were used in a number of laptops and at least one mass
market PCI ethernet card.

Signed-off-by: Mark Brown <broonie@sirena.org.uk>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>