staging: iio: light: isl29018/28: remove I2C_CLASS_HWMON .class setting

I2C_CLASS_HWMON is for a hardware monitoring chip wanting
auto-detection.  IIO drivers don't typically use .class.
Remove it.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>