Currently each onboard_hub platform device owns an 'attach' work,
which is scheduled when the device probes. With this deadlocks
have been reported on a Raspberry Pi 3 B+ [1], which has nested
onboard hubs.

The flow of the deadlock is something like this (with the onboard_hub
driver built as a module) [2]:

- USB root hub is instantiated
- core hub driver calls onboard_hub_create_pdevs(), which creates the
  'raw' platform device for the 1st level hub
- 1st level hub is probed by the core hub driver
- core hub driver calls onboard_hub_create_pdevs(), which creates
  the 'raw' platform device for the 2nd level hub

- onboard_hub platform driver is registered
- platform device for 1st level hub is probed
  - schedules 'attach' work
- platform device for 2nd level hub is probed
  - schedules 'attach' work

- onboard_hub USB driver is registered
- device (and parent) lock of hub is held while the device is
  re-probed with the onboard_hub driver

- 'attach' work (running in another thread) calls driver_attach(), which
   blocks on one of the hub device locks

- onboard_hub_destroy_pdevs() is called by the core hub driver when one
  of the hubs is detached
- destroying the pdevs invokes onboard_hub_remove(), which waits for the
  'attach' work to complete
  - waits forever, since the 'attach' work can't acquire the device lock

Use a single work struct for the driver instead of having a work struct
per onboard hub platform driver instance. With that it isn't necessary
to cancel the work in onboard_hub_remove(), which fixes the deadlock.
The work is only cancelled when the driver is unloaded.

[1] https://lore.kernel.org/r/d04bcc45-3471-4417-b30b-5cf9880d785d@i2se.com/
[2] https://lore.kernel.org/all/Y6OrGbqaMy2iVDWB@google.com/

Cc: stable@vger.kernel.org
Fixes: 8bc06364 ("usb: misc: Add onboard_usb_hub driver")
Link: https://lore.kernel.org/r/d04bcc45-3471-4417-b30b-5cf9880d785d@i2se.com/
Link: https://lore.kernel.org/all/Y6OrGbqaMy2iVDWB@google.com/Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Link: https://lore.kernel.org/r/20230110172954.v2.2.I16b51f32db0c32f8a8532900bfe1c70c8572881a@changeidSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>