i2c: mxs: Fix type of error code

cmd_err is used to handle error code, so it should not be unsigned.

This fixes the following warning when building with W=1 option:

drivers/i2c/busses/i2c-mxs.c: In function 'mxs_i2c_xfer_msg':
drivers/i2c/busses/i2c-mxs.c:331:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: stable@kernel.org

drivers/i2c/busses/i2c-mxs.c

@@ -127,7 +127,7 @@ struct mxs_i2c_dev {
 	struct device *dev;
 	void __iomem *regs;
 	struct completion cmd_complete;
-	u32 cmd_err;
+	int cmd_err;
 	struct i2c_adapter adapter;
 	const struct mxs_i2c_speed_config *speed;