staging: comedi: s626: fix cmd->stop_arg validation for stop_src == TRIG_COUNT

The only limit for the stop_arg with a stop_src == TRIG_COUNT is that there is at
least 1 scan in order to prevent an empty acquisition. The max test is bogus.

Fix the validation.

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>