staging: comedi: usbduxfast: TRIG_EXT is only valid for cmd->start_src

According to the code, and verified with the author Bernd Porr, the
TRIG_EXT trigger is only valid for the cmd->start_src. Remove this
trigger from the cmd->scan_begin_src and cmd->convert_src.

The only valid scan_begin_src is then TRIG_FOLLOW  and the convert_src
is only TRIG_TIMER. Remove the redundant unique trigger checks in Step
2b of the (*do_cmdtest) as well as any other unnecessary checks of
those trigger sources.

Also remove the incorrect comment for a cmd->stop_src of TRIG_EXT.

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>