iio: adc: exynos: do not rely on 'users' counter in ISR

iio: adc: exynos: do not rely on 'users' counter in ISR

The order in which 'users' counter is decremented vs calling drivers'
close() method is implementation specific, and we should not rely on
it. Let's introduce driver private flag and use it to signal ISR
to exit when device is being closed.

This has a side-effect of fixing issue of accessing inut->users
outside of input->mutex protection.

Reported-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201006215509.GA2556081@dtor-ws
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>