iio: core-trigger: make iio_device_register_trigger_consumer() an int return

Oddly enough the noop function is an int-return. This one seems to be void.
This change converts it to int, because we want to change how groups are
registered. With that change this function could error out with -ENOMEM.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-10-alexandru.ardelean@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>