staging: iio: resolver: ad2s1210: remove config attribute

This removes the config register sysfs attribute.

Writing to the config register directly can be dangerous and userspace
should not need to have to know the register layout. This register
can still be accessed though debugfs if needed.

We can add new attributes to set specific flags in the config register
in the future if needed (e.g. `enable_hysterisis` and
`phase_lock_range`).

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20230929-ad2s1210-mainline-v3-12-fa4364281745@baylibre.com


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