staging: comedi: ni_660x: tidy up ni_660x_dio_insn_bits()

Use some local variables to clarify this function.

This (*insn_bits) function is a bit different from most comedi drivers.
Add some comments to clarify why the shifts are used.

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>