iio: adc: ti-ads1015: fix conversion when CONFIG_PM is not set

To stop conversion ads1015_set_power_state() function call unimplemented
function __pm_runtime_suspend() from pm_runtime_put_autosuspend()
if CONFIG_PM is not set.
In case of CONFIG_PM is not set: __pm_runtime_suspend() returns -ENOSYS,
so ads1015_read_raw() failed because ads1015_set_power_state() returns an
error.

If CONFIG_PM is disabled, there is no need to start/stop conversion.
Fix it by adding return 0 function variant if CONFIG_PM is not set.
Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Fixes: ecc24e72 ("iio: adc: Add TI ADS1015 ADC driver support")
Tested-by: Maxim Kiselev <bigunclemax@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ti-ads1015.c

@@ -316,6 +316,7 @@ static const struct iio_chan_spec ads1115_channels[] = {
 	IIO_CHAN_SOFT_TIMESTAMP(ADS1015_TIMESTAMP),
 };
 
+#ifdef CONFIG_PM
 static int ads1015_set_power_state(struct ads1015_data *data, bool on)
 {
 	int ret;
@@ -333,6 +334,15 @@ static int ads1015_set_power_state(struct ads1015_data *data, bool on)
 	return ret < 0 ? ret : 0;
 }
 
+#else /* !CONFIG_PM */
+
+static int ads1015_set_power_state(struct ads1015_data *data, bool on)
+{
+	return 0;
+}
+
+#endif /* !CONFIG_PM */
+
 static
 int ads1015_get_adc_result(struct ads1015_data *data, int chan, int *val)
 {