USB: usbnet reports minidriver name through ethtool

Update "usbnet" so that ethtool reports the name of the minidriver in use
(e.g. asix, cdc_ether, dm9601, rndis_host) instead of "usbnet".  This is a
better match to how other network drivers work, resolving a minor open issue.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>