staging: comedi: das6402: introduce das6402_ai_set_mode()

Introduce a helper function to set the analog input acquisition mode.
This will be needed when the (*do_cmd) function is completed to support
async commands.

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>