hwmon: (iio_hwmon) Fix null pointer dereference

This patch fixes the null pointer dereference in goto error_release_channels
path when allocate memory for st fails.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>