iio: adc: at91-sama5d2_adc: Fix potential use-after-free in sama5d2_adc driver

at91_adc_interrupt can call at91_adc_touch_data_handler function
to start the work by schedule_work(&st->touch_st.workq).

If we remove the module which will call at91_adc_remove to
make cleanup, it will free indio_dev through iio_device_unregister but
quite a bit later. While the work mentioned above will be used. The
sequence of operations that may lead to a UAF bug is as follows:

CPU0 CPU1

| at91_adc_workq_handler
at91_adc_remove |
iio_device_unregister(indio_dev) |
//free indio_dev a bit later |
| iio_push_to_buffers(indio_dev)
| //use indio_dev

Fix it by ensuring that the work is canceled before proceeding with
the cleanup in at91_adc_remove.

Fixes: 23ec2774f1cc ("iio: adc: at91-sama5d2_adc: add support for position and pressure channels")
Signed-off-by: Pei Xiao <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/at91-sama5d2_adc.c

···
 	struct at91_adc_state *st = iio_priv(indio_dev);
 
 	iio_device_unregister(indio_dev);
+	cancel_work_sync(&st->touch_st.workq);
 
 	at91_adc_dma_disable(st);