staging: comedi: ni_6527: factor out the code that sets the filter interval

staging: comedi: ni_6527: factor out the code that sets the filter interval

To clarify the digital input subdevice (*insn_config) a bit, factor out the
code that sets the filter interval for deglitching the digital inputs.

Also, rename the CamelCase define used for the filter interval registers.

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>