iio: adc: at91_adc: change at91_ts_sample to return void

The return value of at91_ts_sample() is never checked by its caller.
Change the return type to void to make this explicit. The error
conditions are already logged via dev_err() which provides sufficient
visibility into hardware issues.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>