staging: comedi: comedi_8254.h: make self-reliant

The Comedi "comedi_8254.h" header file is included by various Comedi
drivers with timer/counters based on the 8254 chip.  The drivers do not
compile cleanly if this header file is included first.  It uses pointers
to the `struct comedi_device`, `struct comedi_subdevice`, and `struct
comedi_insn` structures in various function prototypes, so declare those
as incomplete types.  It use the `bool` type, so include
<linux/types.h>.  It also uses the `__iomem` tag, but that seems to be
taken care of indirectly by including <linux/types.h>.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>