Staging: iio: adt7316: Add an extra check for 'ret' equals to 0

ret = 0 indicates a case of no error but no data read from
the bus which is an invalid case. This case doesn't ever happen
in reality. It should perhaps be handled for correctness though.

Signed-off-by: Shreeya Patel <shreeya.patel23498@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>