powerpc/fsl_msi: add a const qualifier

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

	arch/powerpc/sysdev/fsl_msi.c: In function 'fsl_of_msi_probe':
	arch/powerpc/sysdev/fsl_msi.c:379:11: error: assignment discards 'const' qualifier from pointer target type [-Werror]

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