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-4.19 commit 104eef6e1fe8 ("platform: msm: Remove unused local count
variable").

Change-Id: Id5216e3de9b2edb8d1fa79d0b53e172fcb451ec6
Signed-off-by: Jack Pham <jackp@codeaurora.org>