canbusutil: directly pass QCanBusFrame to parsePayloadField()

It makes no sense to return all information in different variables and
assemble the frame in the caller, just let this method put everything
into the frame. While at it, give the method a more sensible name.

Change-Id: If7e7d467ac4c43d2b2cf245c95e8fe99551aa242
Reviewed-by: Andreas Wilhelm <aw@emlix.com>
Reviewed-by: André Hartmann <aha_1980@gmx.de>