iio: Wakeup poll and blocking reads when the device is unregistered

Once the device has been unregistered there won't be any new data no matter how
long a userspace application waits, so we might as well wake them up and let
them know.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --cc drivers/iio/iio_core.h
Simple merge

diff --cc drivers/iio/industrialio-buffer.c
Simple merge

diff --cc drivers/iio/industrialio-core.c
Simple merge

diff --cc drivers/iio/industrialio-event.c
Simple merge