USB: serial/kobil_sct, fix potential tty NULL dereference

Make sure that we check the return value of tty_port_tty_get.
Sometimes it may return NULL and we later dereference that.

The only place here is in kobil_read_int_callback, so fix it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>