usb: gadget: Add snapshot of USB RMNET Function driver

This change adds USB RMNET function driver which allows communication
between USB BAM and IPA BAM for RMNET and DPL functionalities over USB.

This snapshot is taken as of msm-4.14 'commit 973e6f5c37fc (ARM: dts:
msm: support A/B OTA for LW.2.0 ")'.

This change adding changes to use cdev device instead of misc device
and also fixes different coding style related warnings.

Change-Id: I899cb02386c0ce63212989710ad2fe111da63ac8
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>