usbnet: improve generic ethtool support

This adds generic support for the ethtool commands get_settings,
set_settings, get_link and nway_reset to usbnet. These are now
implemented using mii functions when a low-level driver supports
mdio_read/mdio_write and does not override the usbnet ethtool
commands with its own.

Currently, this applies to the asix and the mcs7830 drivers.
I have tested it on mcs7830.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: David Hollis <dhollis@davehollis.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>