ASoC: rcar: mark device data as constant

A driver's device data should and can be const. This is a follow-up on
commit 33187fb4

 (ASoC: rsnd: constify of_device_id array) which
marked the of_device_id as const.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>