staging: comedi: usbduxsigma: don't clobber ao_timer in command test

commit c04a1f17 upstream.

`devpriv->ao_timer` is used while an asynchronous command is running on
the AO subdevice.  It also gets modified by the subdevice's `cmdtest`
handler for checking new asynchronous commands,
`usbduxsigma_ao_cmdtest()`, which is not correct as it's allowed to
check new commands while an old command is still running.  Fix it by
moving the code which sets up `devpriv->ao_timer` into the subdevice's
`cmd` handler, `usbduxsigma_ao_cmd()`.

Note that the removed code in `usbduxsigma_ao_cmdtest()` checked that
`devpriv->ao_timer` did not end up less that 1, but that could not
happen due because `cmd->scan_begin_arg` or `cmd->convert_arg` had
already been range-checked.

Also note that we tested the `high_speed` variable in the old code, but
that is currently always 0 and means that we always use "scan" timing
(`cmd->scan_begin_src == TRIG_TIMER` and `cmd->convert_src == TRIG_NOW`)
and never "convert" (individual sample) timing (`cmd->scan_begin_src ==
TRIG_FOLLOW` and `cmd->convert_src == TRIG_TIMER`).  The moved code
tests `cmd->convert_src` instead to decide whether "scan" or "convert"
timing is being used, although currently only "scan" timing is
supported.

Fixes: fb1ef622

 ("staging: comedi: usbduxsigma: tidy up analog output command support")
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: Bernd Porr <mail@berndporr.me.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>