comedi: dt2815: add hardware detection to prevent crash

In the Linux kernel, the following vulnerability has been resolved:

comedi: dt2815: add hardware detection to prevent crash

The dt2815 driver crashes when attached to I/O ports without actual

hardware present. This occurs because syzkaller or users can attach

the driver to arbitrary I/O addresses via COMEDI_DEVCONFIG ioctl.

When no hardware exists at the specified port, inb() operations return

0xff (floating bus), but outb() operations can trigger page faults due

to undefined behavior, especially under race conditions:

BUG: unable to handle page fault for address: 000000007fffff90

#PF: supervisor write access in kernel mode

#PF: error_code(0x0002) - not-present page

RIP: 0010:dt2815_attach+0x6e0/0x1110

Add hardware detection by reading the status register before attempting

any write operations. If the read returns 0xff, assume no hardware is

present and fail the attach with -ENODEV. This prevents crashes from

outb() operations on non-existent hardware.