Add RmNet support for LARA-L6.

LARA-L6 module can be configured (by AT interface) in three different
USB modes:
* Default mode (Vendor ID: 0x1546 Product ID: 0x1341) with 4 serial
interfaces
* RmNet mode (Vendor ID: 0x1546 Product ID: 0x1342) with 4 serial
interfaces and 1 RmNet virtual network interface
* CDC-ECM mode (Vendor ID: 0x1546 Product ID: 0x1343) with 4 serial
interface and 1 CDC-ECM virtual network interface

In RmNet mode LARA-L6 exposes the following interfaces:
If 0: Diagnostic
If 1: AT parser
If 2: AT parser
If 3: AT parset/alternative functions
If 4: RMNET interface
Signed-off-by: Davide Tronchin <davide.tronchin.94@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>