net: hso: fix NULL-deref on disconnect regression

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

net: hso: fix NULL-deref on disconnect regression

Commit 8a12f8836145 ("net: hso: fix null-ptr-deref during tty device

unregistration") fixed the racy minor allocation reported by syzbot, but

introduced an unconditional NULL-pointer dereference on every disconnect

instead.

Specifically, the serial device table must no longer be accessed after

the minor has been released by hso_serial_tty_unregister().