i2c/mpc: add a const qualifier

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

	drivers/i2c/busses/i2c-mpc.c: In function 'fsl_i2c_probe':
	drivers/i2c/busses/i2c-mpc.c:650:31: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]

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