iio:trigger: Introduce the use of devm_kzalloc

This patch introduces the use of the managed version of kzalloc and
removes the kfrees in the probe and remove functions. More return
paths are added and the labels are renamed to order them.

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>