staging: comedi: use comedi_alloc_devpriv()

Use the helper function to allocate memory and set the comedi_device
private data pointer.

This removes the dependency on slab.h from most of the drivers so
remove the global #include in comedidev.h and the local #include
in some of the drivers.

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