net/usb: add sierra_net.c driver

Re-submitted based on comments from netdev community.
Summary of the changes:
1. Improved error handling.
2. Added the missing timeout arguments to usb_control_msg().

The following is a new Linux driver which exposes certain models of Sierra
Wireless modems to the operating system as Network Interface Cards (NICs).

This driver requires a version of the sierra.c driver which supports
blacklisting to work properly. The blacklist in sierra.c rejects the interfaces
claimed by sierra_net.c. Likewise, the sierra_net.c driver only accepts
(i.e. whitelists) the interface(s) used for USB-to-WWAN traffic.
The version of sierra.c which supports blacklisting is
available from the sierra wireless knowledge base page for older kernels. It is
also available in Linux kernel starting from version 2.6.31.

This driver works with all Sierra Wireless devices configured with PID=68A3
like USB305, USB306 provided the corresponding firmware version is I2.0
(for USB305) or M3.0 (for USB306) and later.
This driver will not work with earlier firmware versions than the ones shown
above. In this case the driver will issue an error message indicating
incompatibility and will not serve the device's USB-to-WWAN interface.

Sierra_net.c sits atop a pre-existing Linux driver called usbnet.c.
A series of hook functions are provided in sierra_net.c which are called by
usbnet.c in response to a particular condition such as receipt or transmission
of a data packet. As such, usbnet.c does most of the work of making
a modem appear to the system as a network device and for properly exchanging
traffic between the USB subsystem and the Network card interface.
Sierra_net.c is concerned with managing the data exchanged between the
USB-to-WWAN interface and the upper layers of the operating system.

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: Rory Filer <rfiler@sierrawireless.com>
Signed-off-by: David S. Miller <davem@davemloft.net>