USB: usb-skeleton.c: fix blocked forever in skel_read

In skel_read,the reader blocked in wait_for_completion before submit
bulk in urb.
Using processed_urb is for retaining the completion in the case that
previous interruptible wait in skel_read was interrupted and complete
before next skel_read.  Replacing completion with waitqueue can avoid
working around the counting nature of completions
and fix the bug.

Signed-off-by: Du Xing <duxing2007@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>