iio: chemical: atlas-ezo-sensor: move ret variable scope in atlas_ezo_read_raw()

Move ret variable to the IIO_CHAN_INFO_RAW switch since currently
only used within that scope.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>