staging: comedi: ni_65xx: filter interval register is 32-bit

According to the register programming manual, the filter interval
register is 32-bit. Fix the writes to this register.

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>