net: cdc_ether: record speed in status method

Until very recently, the usbnet framework only had support functions
for devices which reported the link speed by explicitly querying the
PHY over a MDIO interface. However, the cdc_ether devices send
notifications when the link state or link speeds change and do not
expose the PHY (or modem) directly.

Support funtions (e.g. usbnet_get_link_ksettings_internal()) to directly
query state recorded by the cdc_ether driver were added in a previous patch.

Instead of cdc_ether spewing the link speed into the dmesg buffer,
record the link speed encoded in these notifications and tell the
usbnet framework to use the new functions to get link speed/state.

User space can now get the most recent link speed/state using ethtool.

v4: added to series since cdc_ether uses same notifications
    as cdc_ncm driver.

Signed-off-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>