macintosh/mediabay: make data used as *of_device_id.data const

Since of_device_id.data is declared as a pointer to const data a few
more consts can be added in this driver.

[ukl: split Arnd's patch by driver]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>