USB: core: of: document reference taken by companion helper

Document that the new companion-device lookup helper takes a reference
to the companion device which needs to be dropped after use.

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>