iio: adc: ad4851: Fix signedness bug in ad4851_calibrate()

iio: adc: ad4851: Fix signedness bug in ad4851_calibrate()

The "c" variable is used to store error codes from ad4851_find_opt()
so it has to be signed for the error handling to work.  Change it
to type int.

Fixes: 6250803fe2ec ("iio: adc: ad4851: add ad485x driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/f5e260e9-d7a8-4dae-b7ea-f1bbb1760e60@stanley.mountain
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>