iio: adc: ad_sigma_delta: Remove unused variable 'ret'

Fixes the following W=1 kernel build warning(s):

 drivers/iio/adc/ad_sigma_delta.c: In function ‘ad_sd_trigger_handler’:
 drivers/iio/adc/ad_sigma_delta.c:405:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
 405 | int ret;
 | ^~~

Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>