Documentation/3c509: document ethtool support

3c509 was changed to support ethtool in 2002, making the 'xcvr' module
parameter obsolete in most cases.  More recently 3c509 was converted
to the modern driver model and this parameter was removed.  Fix the
documentation to refer to ethtool rather than the module parameter.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>