Instead of just returning an error code, add an error message as well.
While at it, simplify the code and use a common return path.
Upon deferral this also nicely lists the following message in
/sys/kernel/debug/devices_deferred:
adc     iio_hwmon: Failed to get channels
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://lore.kernel.org/r/20230131103359.625081-1-alexander.stein@ew.tq-group.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>