staging: comedi: drivers: introduce comedi_alloc_devpriv()

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

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>