iio: adc: ad7606: Fix incorrect type for error return variable

iio: adc: ad7606: Fix incorrect type for error return variable

The variable ret is declared as unsigned int but is used to store return
values from functions returning int, which may be negative error codes.

Change ret from unsigned int to int.

Fixes: 849cebf8dc67 ("iio: adc: ad7606: Add iio-backend support")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>