iio: adc: mt6360: Drop an incorrect __maybe_unused marking.

Given the struct platform_driver has one of it's elements assigned to
point to the of_device_id table, it is never going to be unused.

Drop the marking.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Gene Chen <gene_chen@richtek.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220807162121.862894-1-jic23@kernel.org