staging: comedi: s626: 'ai_sample_count' is always 'stop_arg'

When the cmd->stop_src == TRIG_COUNT, the 'ai_sample_count' is the cmd->stop_arg.
For a stop_src == TRIG_NONE the 'ai_sample_count' is currently set to 1 but the
driver never actually uses the value.

Simplify the 'stop_count' initialization by just setting it to the cmd->stop_arg.

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>