staging:iio:ad7606: Remove default device configuration from platform data

While for some very selected setups it might be useful to be able to
provide default configuration data via the platform data, generally this
becomes very impractical as the number of configuration options increases.
So the general policy is to use the power-on default values of the device
and let the application using the device configure it according to its
needs.

Implement this scheme for the ad7606 driver by removing support for
specifying a default configuration via the platform data.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>