iio: temperature: max30208: Remove an unused field in struct max30208_data

In "struct max30208_data", the 'indio_dev' field is unused.
Remove it.

Found with cppcheck, unusedStructMember.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/73f9f540ecdc7c10e833e6fc782324ae7d34ba9c.1714555144.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>