net: usb: Add the USB RmNet host driver

Add support for RmNet networking over USB. This is based on a
snapshot from kernel msm-3.10 'commit ee337c6f350c
("net: rmnet_usb: Add sleep state extended IOCTL")' with changes
to remove ether mode support and some code cleanup.
This driver is used for only DATA message transactions between
host and device.

Change-Id: I6919ddf67613b1434adb78587e3a523897c2b5c6
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>