staging: iio: adt7316: drop unnecessary initialization of variables

Initialization is unnecessary when the variable is written before it is
read. There were some occasions in which the driver would initialize `ret'
during declaration without need.

Purely a cosmetic change with no functional impact.

Signed-off-by: Hardik Singh Rathore <hardiksingh.k@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>