staging: comedi: ni_mio_common: tidy up AO scan begin

The (*do_cmdtest) validates that the cmd->scan_begin_src will only be
TRIG_TIMER or TRIG_EXT. Refactor ni_ao_cmd_set_update() and remove the
BUG() which can never occur.

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>