iio: adc: ad7091r5: Simplify driver_data handling

iio: adc: ad7091r5: Simplify driver_data handling

The driver only supports a single device type. So the return value of
i2c_get_match_data() is already known and can just be hardcoded without
type casting. While at it also remove the unused .data of the
single of_device_id entry.

Signed-off-by: Uwe Kleine-König (The Capable Hub) <u.kleine-koenig@baylibre.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>