iio: magn: st_magn: use devm_iio_triggered_buffer_setup() for buffer

iio: magn: st_magn: use devm_iio_triggered_buffer_setup() for buffer

The st_magn_allocate_ring() function calls iio_triggered_buffer_setup() to
allocate a triggered buffer.

But the same can be done with devm_iio_triggered_buffer_setup() and then
the st_magn_common_remove() no longer needs to manually deallocate it.

We know that the parent of the IIO device is used to manage other instances
of the devm unwind, so it can be used in the st_magn_allocate_ring() as
well.

This change also removes some omitted st_magn_{probe,remove}_trigger()
inline hooks.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210720074642.223293-3-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>