staging: mt7621-mmc: Fix: copy_from_user() returns a positive value

copy_from_user() returns a positive value in case of an error, to fix
this the check is turned around, also a better return value is chosen.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-mmc/dbg.c

@@ -265,9 +265,8 @@ static ssize_t msdc_debug_proc_write(struct file *file,
 	if (count > 255)
 		count = 255;
 
-	ret = copy_from_user(cmd_buf, buf, count);
-	if (ret < 0)
-		return -1;
+	if (copy_from_user(cmd_buf, buf, count))
+		return -EFAULT;
 
 	cmd_buf[count] = '\0';
 	printk("msdc Write %s\n", cmd_buf);