spi/spi-omap2-mcspi: add a const qualifier

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

	drivers/spi/spi-omap2-mcspi.c: In function 'omap2_mcspi_probe':
	drivers/spi/spi-omap2-mcspi.c:1118: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>