mmc: Replace printf with debug call for timeouts in the i.MX28 mxs driver

This change replaces printf() with debug() for the notification about
commands timeouts.

This is done on purpose (also other drivers use such approach - dw_mmc.c,
mvebu_mmc.c), as the mmc core code (mmc.c) uses timeouts to assess if one
is using sd card or eMMC device.
In such situation timeout is a some kind of a "normal" behavior and there
shall not be any output to the console.

There is no impact on boot time for boards using this driver (even in SPL)
when two extra timeouts are returned (no SD card present, only eMMC
available).

Boot time tested with grabserial:
sudo grabserial -b 115200 -d /dev/ttyUSB1 -e 30 -t -m "^U-Boot SPL*"
Signed-off-by: NLukasz Majewski <lukma@denx.de>

drivers/mmc/mxsmmc.c

@@ -432,7 +432,7 @@ mxsmmc_send_cmd(struct udevice *dev, struct mmc_cmd *cmd, struct mmc_data *data)
 
 	/* Check command timeout */
 	if (reg & SSP_STATUS_RESP_TIMEOUT) {
-		printf("MMC%d: Command %d timeout (status 0x%08x)\n",
+		debug("MMC%d: Command %d timeout (status 0x%08x)\n",
 		      devnum, cmd->cmdidx, reg);
 		return -ETIMEDOUT;
 	}