staging: comedi: have comedi_set_spriv() allocate the memory

As suggested by Ian Abbott, comedi_set_spriv() can only be used to
set the subdevice->private pointer to something that can be kfree()'d.
Rename the function to comedi_alloc_spriv() and have it kzalloc() the
memory as well as set the private pointer. This saves a function call
in the drivers and avoids the possibility of incorrectly calling
comedi_set_spriv() for some pointer that is not meant to be kfree()'d.

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>