sfc: remove efx_ethtool_nway_reset()

An MDIO-based n-way restart does not make sense for any of the NICs
 supported by this driver, nor for the coming EF100.
Unlike on Falcon (which was already split off into a separate driver),
 the PHY on all of Siena, EF10 and EF100 is managed by MC firmware.
While Siena can talk to the PHY over MDIO, doing so for anything other
 than debugging purposes (mdio_mii_ioctl) is likely to confuse the
 firmware.
(According to the SFC firmware team, this support was originally added
 to the Siena driver early in the development of that product, before
 it was decided to have firmware manage the PHY.)

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>