iio: adc: ti-ads1015: use devm_iio_triggered_buffer_setup

Use devm_iio_triggered_buffer_setup to simplify the error path in the
probe() and remove() function.

This changes the remove order, but the end result of remove function
actually does the reverse of probe.

Cc: Daniel Baluta <daniel.baluta@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>