staging: comedi: dmm32at: introduce dmm32at_ai_set_chanspec()

Introduce a helper function to set the analog input channel scan and range
control registers. Use the new helper in the analog input (*insn_read) and
(*do_cmd) functions.

The extra sanity checks in the (*do_cmd) are not necessary. The cmd->chanlist
will always be valid and the (*do_cmdtest) validates the channel list in
Step 5.

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>