staging: comedi: cb_das16_cs: fix ai mux register programming

The ai mux register is used program the hi/lo channels in a scan.
According to the user manual, the hi and lo channels should be
the same  to sample one channel.

Introduce some macros to set the appropriate bits in the ai mux
register and fix the ai (*insn_read).

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>