usb: gadget: Add snapshot of USB DIAG function driver

This change adds USB DIAG function driver to support diagnostics
functionality with USB transport. This snapshot is taken as of
msm-4.9 commit 9e420a36242d ("ARM: dts: msm: Add dtsi entries of USB
for SDM670").

This includes a minor fix for the following checkpatch warning:

	WARNING: Prefer using '"%s...", __func__' to using
	'diag_function_disable', this function's name, in a string

Change-Id: Id04d19d8e461b07f2b0760b573cba417bf59181a
Signed-off-by: Jack Pham <jackp@codeaurora.org>