iio: Return -ENODEV for file operations if the device has been unregistered

If the IIO device has been unregistered return -ENODEV for any further file
operations like read() and ioctl(). This avoids userspace being able to grab new
references to the device.

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

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