USB: serial: ftdi_sio: simplify TIOCGSERIAL permission check

Changing the deprecated custom_divisor field is an unprivileged
operation so after verifying that flag field does not contain any
privileged changes both updates can be carried out by any user.

Combine the two branches and drop the erroneous comment.

Note that private flags field is only used for ASYNC flags so there's no
need to try to retain any other bits when updating the flags.
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ftdi_sio.c

@@ -1496,27 +1496,16 @@ static int set_serial_info(struct tty_struct *tty,
 	mutex_lock(&priv->cfg_lock);
 	old_priv = *priv;
 
-	/* Do error checking and permission checking */
-
 	if (!capable(CAP_SYS_ADMIN)) {
 		if ((ss->flags ^ priv->flags) & ~ASYNC_USR_MASK) {
 			mutex_unlock(&priv->cfg_lock);
 			return -EPERM;
 		}
-		priv->flags = ((priv->flags & ~ASYNC_USR_MASK) |
-			       (ss->flags & ASYNC_USR_MASK));
-		priv->custom_divisor = ss->custom_divisor;
-		goto check_and_exit;
 	}
 
-
-	/* Make the changes - these are privileged changes! */
-
-	priv->flags = ((priv->flags & ~ASYNC_FLAGS) |
-					(ss->flags & ASYNC_FLAGS));
+	priv->flags = ss->flags & ASYNC_FLAGS;
 	priv->custom_divisor = ss->custom_divisor;
 
-check_and_exit:
 	write_latency_timer(port);
 
 	if ((priv->flags ^ old_priv.flags) & ASYNC_SPD_MASK ||