iio: bmi160: use all devm functions in probe

Currently, we're using the devm version of some but not all functions.
Switch to the devm version of iio_triggered_buffer_setup and
iio_device_register to simplify the code a bit and decrease the chance of
bugs.

Signed-off-by: Martin Kelly <martin@martingkelly.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>