staging: comedi: das16: introduce das16_ai_set_mux_range()

Introduce a helper function to program the mux and gain for analog input
single channel (*insn_read) and multi-channel (*do_cmd) operations.

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>