staging: comedi: drivers: ni_mio_common: add names to function pointer parameters.

Checkpatch.pl complains about packbits function pointer that lacks
parameters name.

Add parameter names to packbits function pointer.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>