Phonet: USB CDC Phonet function for gadget framework

This implements the Nokia vendor-specific communication device class
function to exchange Phonet messages over USB. This function is already
found in the "PC suite" USB profile of (non-Linux) Nokia handsets.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>