iio: adc: ti-ads1015: add helper to set conversion mode

This adds a helper function to set conversion mode as there are a fair
number of users.

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>