usb: gadget: Add snapshot of USB CDEV function driver

This change adds USB CDEV function driver which creates USB character
device to provide DUN and NMEA functionality. This snapshot is taken
as of msm-4.4 commit 4195f84bde61 ("qcom-charger: smb2: add support
of PMFALCON").

It fixes couple of compilation errors as below:
1. usb_assign_descriptors() definition change
2. warning: cast to pointer from integer of different size
[-Wint-to-pointer-cast] with usb_cser_alloc_chardev_region() API

It also fixes multiple warnings related to style and usage of unsigned.

Change-Id: I028e5fd23e88d5a247a3f217530d3263245c08d3
Signed-off-by: Mayank Rana <mrana@codeaurora.org>