staging: comedi: ni_labpc: clarify the cmd->start_arg validation

This driver supports cmd->start_src sources of TRIG_NOW and TRIG_EXT.
Refactor the (*do_cmdtest) to clarify the trivial validation of the
cmd->start_arg.

This also fixes a bug, the cmd->start_src has the trigger not the
cmd->start_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>