mmc: debugfs: remove BUG_ON from mmc_ext_csd_open

Return error value for file_operations callback instead
of triggering BUG_ON which is meaningless. Personally I
don't believe n != EXT_CSD_STR_LEN could happen. Anyway,
propagate the error to the caller.
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/debugfs.c

@@ -321,7 +321,11 @@ static int mmc_ext_csd_open(struct inode *inode, struct file *filp)
 	for (i = 0; i < 512; i++)
 		n += sprintf(buf + n, "%02x", ext_csd[i]);
 	n += sprintf(buf + n, "\n");
-	BUG_ON(n != EXT_CSD_STR_LEN);
+
+	if (n != EXT_CSD_STR_LEN) {
+		err = -EINVAL;
+		goto out_free;
+	}
 
 	filp->private_data = buf;
 	kfree(ext_csd);