tty: USB: serial/mct_u232, fix tty refcnt

Stanse found a tty refcnt leak in read_int_callback. In fact
it's handled wrong altogether. tty_port_tty_get can return NULL
and it's not checked in that manner.

Fix that by checking the tty_port_tty_get retval and put tty kref
properly.

http://stanse.fi.muni.cz/

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>