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

The Comedi "comedi_isadma.h" header is included by the source for the
"comedi_isadma" helper module and other modules that use it.  It does
not compile cleanly when it is the first header file included.  It uses
the `dma_addr_t` type, so include <linux/types.h> to declare it.  (Also,
that indirectly takes care of the use of `NULL`.)  It uses `struct
comedi_device *` in various function prototypes, so add an incomplete
declaration of `struct comedi_device`.

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>