iio: sx9500: Use complete() instead of complete_all()

iio: sx9500: Use complete() instead of complete_all()

There is only one waiter for the completion, therefore there
is no need to use complete_all(). Let's make that clear by
using complete() instead of complete_all().

The usage pattern of the completion is:

waiter context                          waker context

sx9500_read_proximity()
  sx9500_inc_chan_users()
  sx9500_inc_data_rdy_users()
  wait_for_completion_interruptible()

                                        s9500_irq_thread_handler()
                                          complete()

  reinit_completion()

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>