staging: comedi: ni_65xx: factor input filter disable out of (*auto_attach)

For aesthetics, factor the code that disables the input filters out of
ni_65xx_auto_attach().

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>