iio: adc: rcar-gyroadc: Use of_device_get_match_data() helper

Use the of_device_get_match_data() helper instead of open coding.
Note that the rcar-gyroadc driver is used with DT only, so there's
always a valid match.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>