i2c/i2c-omap: add a const qualifier

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

	drivers/i2c/busses/i2c-omap.c: In function 'omap_i2c_probe':
	drivers/i2c/busses/i2c-omap.c:1025: warning: assignment discards qualifiers from pointer target type

Reviewed-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>