USB: gadget: Document that certain ep operations can be called in interrupt context

This documentation patch specifies that certain USB gadget endpoint
operations may be called in interrupt context:

	usb_ep_queue, usb_ep_dequeue, usb_ep_set_halt,
	usb_ep_clear_halt, usb_ep_set_wedge, usb_ep_fifo_status,
	and usb_ep_fifo_flush;

while others must be called in process context:

	usb_ep_enable and usb_ep_disable.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>