USB: serial: io_ti: fix another NULL-deref at open

USB: serial: io_ti: fix another NULL-deref at open

commit 4f9785cc99feeb3673993b471f646b4dbaec2cc1 upstream.

In case a device is left in "boot-mode" we must not register any port
devices in order to avoid a NULL-pointer dereference on open due to
missing endpoints. This could be used by a malicious device to trigger
an OOPS:

Unable to handle kernel NULL pointer dereference at virtual address 00000030
...
[<bf0caa84>] (edge_open [io_ti]) from [<bf0b0118>] (serial_port_activate+0x68/0x98 [usbserial])
[<bf0b0118>] (serial_port_activate [usbserial]) from [<c0470ca4>] (tty_port_open+0x9c/0xe8)
[<c0470ca4>] (tty_port_open) from [<bf0b0da0>] (serial_open+0x48/0x6c [usbserial])
[<bf0b0da0>] (serial_open [usbserial]) from [<c0469178>] (tty_open+0xcc/0x5cc)

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2:
 - No heartbeat_work to initialise earlier
 - No separate port_probe and port_remove operations, so add check for null
   port pointers in edge_release()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>