ANDROID: usb: gadget: f_accessory: Migrate to USB_FUNCTION API

This patch adds support to use Android accessory
gadget function through the
DECLARE_USB_FUNCTION_INIT interface.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Change-Id: Ib352752d5bc905fa1df9049b53eabf1294930db7

[AmitP: Folded following android-4.9 commit changes into this patch
        Parts of e45c769fa7af ("ANDROID: usb: gadget: cleanup: fix unused variable and function warnings")
        a0a752add9b5 ("ANDROID: usb: gadget: f_accessory: remove duplicate endpoint alloc")
        Parts of 051584e76d12 ("ANDROID: usb: gadget: function: cleanup: Add blank line after declaration")]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>