i2c: imx: make bitrate an u32 type

sparse found this assignment of u32 to an int. Fix it:

drivers/i2c/busses/i2c-imx.c:540:56: warning: incorrect type in argument 3 (different signedness)

and also fix the type in platform_data. All current users use values
which fit into the old and new type, so it is a safe change.

Signed-off-by: Wolfram Sang <wolfram@the-dreams.de>
Reviewed-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>