staging: comedi: hwdrv_apci3120: cmd->convert_src is always TRIG_TIMER

The (*do_cmdtest) validates that the cmd->convert_src can only be
TRIG_TIMER. Remove the unnecessay checks.

THis also simplifies the (*do_cmd) a bit since "mode 3" (whatever that
is) is never a valid mode.

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>