iio: adc: ti-ads1015: don't return invalid value from buffer setup callbacks

pm_runtime_get_sync() and pm_runtime_put_autosuspend() return 0 on
success, 1 if the device's runtime PM status was already requested status
or error code on failure.  So a positive return value doesn't indicate an
error condition.

However, any non-zero return values from buffer preenable and postdisable
callbacks are recognized as an error and this driver reuses the return
value from pm_runtime_get_sync() and pm_runtime_put_autosuspend() in
these callbacks.  This change fixes the false error detections.

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