staging: comedi: ni_65xx: cleanup INSN_CONFIG_FILTER handling

The INSN_CONFIG_FILTER instruction is used to set the deglitch filter
interval used to debounce the input channels.

Absorb the helper function into the (*insn_config) function and refactor
the code to not require the 'filter_interval' and 'filter_enable' members
in the private data.

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>