iio: adc: twl6030-gpadc: fix error return code in twl6030_gpadc_read_raw()

Fix to return -EINTR in the error handling case instead
of 0 (ret is assigned after goto, which has no effect),
as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>