netdev: Use default implementation of ethtool_ops::get_link where possible

Various drivers are using implementations of ethtool_ops::get_link
that are equivalent to the default ethtool_op_get_link().  Change
them to use that instead.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>