usb: gadget: udc-core: move sysfs_notify() to a workqueue

usb_gadget_set_state() will call sysfs_notify()
which might sleep. Some users might want to call
usb_gadget_set_state() from the very IRQ handler
which actually changes the gadget state.

Instead of having every UDC driver add their own
workqueue for such a simple notification, we're
adding it generically to our struct usb_gadget,
so the details are hidden from all UDC drivers.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Git-commit: 5702f75375aa9ecf8ad3431aef3fe6ce8c8dbd15
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git


Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>