This change converts the st_sensors_allocate_trigger() to use
device-managed functions.

The parent device of the IIO device object is used. This is based on the
assumption that all other devm_ calls in the ST sensors use this reference.

That makes the st_sensors_deallocate_trigger() function un-needed, so it
can be removed.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210823112204.243255-3-aardelean@deviqon.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>