usbnet: use eth%d name for known ethernet devices

usbnet: use eth%d name for known ethernet devices

The documentation for the USB ethernet devices suggests that
only some devices are supposed to use usb0 as the network interface
name instead of eth0. The logic used there, and documented in
Kconfig for CDC is that eth0 will be used when the mac address
is a globally assigned one, but usb0 is used for the locally
managed range that is typically used on point-to-point links.

Unfortunately, this has caused a lot of pain on the smsc95xx
device that is used on the popular pandaboard without an
EEPROM to store the MAC address, which causes the driver to
call random_ether_address().

Obviously, there should be a proper MAC addressed assigned to
the device, and discussions are ongoing about how to solve
this, but this patch at least makes sure that the default
interface naming gets a little saner and matches what the
user can expect based on the documentation, including for
new devices.

The approach taken here is to flag whether a device might be a
point-to-point link with the new FLAG_POINTTOPOINT setting in
the usbnet driver_info. A driver can set both FLAG_POINTTOPOINT
and FLAG_ETHER if it is not sure (e.g. cdc_ether), or just one
of the two.  The usbnet framework only looks at the MAC address
for device naming if both flags are set, otherwise it trusts the
flag.

Signed-off-by: Arnd Bergmann <arnd.bergmann@linaro.org>
Tested-by: Andy Green <andy.green@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>