USB: gadget: Improve kerneldoc for usb_ep_dequeue()

Commit bf594c10 ("USB: gadget: Document that certain ep operations
can be called in interrupt context") documented that usb_ep_dequeue()
may be called in a non-process context.  It follows that the routine
must not sleep or wait for events.

However, the routine's existing kerneldoc seems to imply that it will
wait until the request being cancelled has fully completed.  This is
not so, and thus the comment needs to be improved.  Misunderstanding
this point may very well have been responsible for a bug recently
uncovered in the f_fs function.

The updated comment explicitly says that the routine may return before
the request's completion handler is called.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: John Stultz <john.stultz@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>