staging:iio:ad7793: Remove unused platform_data from device state struct

The platform data for the device is only used from within the drivers probe
callback, so there is no need to keep it around in the devices state struct.

While we are at it mark the platform data struct as const.

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