mmc/omap_hsmmc: add a const qualifier

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	drivers/mmc/host/omap_hsmmc.c: In function 'omap_hsmmc_probe':
	drivers/mmc/host/omap_hsmmc.c:1808: warning: initialization discards qualifiers from pointer target type

Acked-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>