staging:iio:adc:ad7280a: Make oversampling_ratio a runtime control

Oversampling has nothing directly to do with analog circuits or
similar so belongs in the control of userspace as a policy decision.

The only complexity in here was that the acquisition time needs
updating if this setting is changed at runtime (as oversampling
is time consuming).

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-11-jic23@kernel.org