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

This reverts commit 00426e99.

i2c_smbus_read_byte() returns 0 when a byte with the value 0 is read from
the device. This is a valid read so revert the check for 0.

Signed-off-by: Jeremy Fertic <jeremyfertic@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>