USB: URB documentation: claim complete() will be run with IRQs enabled

There is no good reason to run complete() in hard interrupt
disabled context.

After switch to run complete() in tasklet, we will enable local IRQs
when calling complete() since we can do it at that time.

Even though we still disable IRQs now when calling complete()
in tasklet, the URB documentation is updated to claim complete()
will be run in tasklet context and local IRQs will be enabled, so
that USB drivers can know the change and avoid one deadlock caused
by: assume IRQs disabled in complete() and call spin_lock() to
hold lock which might be acquired in interrupt context.

Current spin_lock() usages in drivers' complete() will be cleaned
up at the same time, and once the cleanup is finished, local IRQs
will be enabled when calling complete() in tasklet.

Also fix description about type of usb_complete_t, and remove the
advice of running completion handler in tasklet for decreasing
system latency.

Cc: Oliver Neukum <oliver@neukum.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>