staging: iio: cdc: ad7746: Remove unnecessary assignment in ad7746_probe()

In the function ad7746_probe(), the initialized value of 'ret' is unused,
because it will be assigned by the function i2c_smbus_write_byte_data(),
thus remove it.

Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210518095647.3008-1-tangbin@cmss.chinamobile.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>