iio: inkern: Replace a FIXME comment by a TODO one

iio: inkern: Replace a FIXME comment by a TODO one

This FIXME comment is more a TODO one.
It is a note when someone will need for this currently unsupported case.

Change from FIXME to TODO.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Suggested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/20230623085830.749991-10-herve.codina@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>