staging: comedi: das16m1: introduce das16m1_ai_set_queue()

Introduce a helper function to handle writing the channel/gain data to
the queue for single channel reads, (*insn_read), and multi-channel
scans, (*do_cmd).

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>