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

`devpriv->ai_timer` is used while an asynchronous command is running on
the AI subdevice.  It also gets modified by the subdevice's `cmdtest`
handler for checking new asynchronous commands
(`usbduxsigma_ai_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->ai_timer` and
`devpriv->ai_interval` into the subdevice's `cmd` handler,
`usbduxsigma_ai_cmd()`.

Note that the removed code in `usbduxsigma_ai_cmdtest()` checked that
`devpriv->ai_timer` did not end up less than than 1, but that could not
happen because `cmd->scan_begin_arg` had already been checked to be at
least the minimum required value (at least when `cmd->scan_begin_src ==
TRIG_TIMER`, which had also been checked to be the case).

Fixes: b986be85 ("staging: comedi: usbduxsigma: tidy up analog input 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>
Cc: <stable@vger.kernel.org> # 3.19 onwards
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>