[PATCH] serial moxa: fix wrong BUG

There is a wrong BUG in mxser_close.

The BUG is triggered when tty->driver_data == NULL, But in fact this is not
a bug, because tty->driver->close is called even when tty->driver->open
fails.

LDD3 tells us to do nothing in such cases.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>