usb: misc: onboard_hub: rename to onboard_dev

This patch prepares onboad_hub to support non-hub devices by renaming
the driver files and their content, the headers and their references.

The comments and descriptions have been slightly modified to keep
coherence and account for the specific cases that only affect onboard
hubs (e.g. peer-hub).

The "hub" variables in functions where "dev" (and similar names) variables
already exist have been renamed to onboard_dev for clarity, which adds a
few lines in cases where more than 80 characters are used.

No new functionality has been added.

Acked-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Javier Carrasco <javier.carrasco@wolfvision.net>
Link: https://lore.kernel.org/r/20240325-onboard_xvf3500-v8-2-29e3f9222922@wolfvision.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>