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