iio: proximity: as3935: fix use-after-free on device remove

This driver's probe() uses a mix of devm_ and non-devm_ functions. This
means that the remove order will not be the exact opposite of the probe
order.

Remove order:
1. remove() executes:
	iio_device_unregister
	iio_triggered_buffer_cleanup
	iio_trigger_unregister
	(A)
2. core frees devm resources in reverse order:
	free_irq
	iio_trigger_free
	iio_device_free

In (A) the trigger has been unregistered, but the irq handler is still
registered and active, so the trigger may still be touched via
interrupt -> as3935_event_work. This is a potential use-after-unregister.

Given that the delayed work is never canceled explicitly, it may run even
after iio_device_free. This is a potential use-after-free.

Solution: convert all probe functions to their devm_ equivalents.
Add a devm callback, called by the core on remove right after irq_free,
which explicitly cancels the delayed work. This will guarantee that all
resources are freed in the correct order.

As an added bonus, some boilerplate code can be removed.

Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>