net: cdc_ncm: 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_ncm 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_ncm driver were added in a previous patch.

So instead of cdc_ncm 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.
Link speed/state is now available via ethtool.

This is especially useful given all current RTL8156 devices emit
a connection/speed status notification every 32ms and this would
fill the dmesg buffer. This implementation replaces the one
recently submitted in de658a19 :
   "net: usb: cdc_ncm: don't spew notifications"

v2: rebased on upstream
v3: changed variable names
v4: rewrote commit message

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Tested-by: Roland Dreier <roland@kernel.org>
Signed-off-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>