staging: comedi: quatech_daqp_cs: ai async command requires a pacer

The ai (*do_cmd) assumes that either the convert_src is TRIG_TIMER or
the scan_begin_src is TRIG_TIMER and always programs the pacer clock
with the appropriate trigger argument.

Add a Step 2b check to the (*do_cmdtest) to ensure that at least one
of the triggers is TRIG_TIMER.

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>