staging: comedi: adl_pci9111: clarify Step 2b of the (*do_cmdtest)

This step of the (*do_cmdtest) verifies that the selected trigger sources
are mutually compatible.

For this driver the scan_begin_src must be TRIG_FOLLOW or the same source
as the convert_src.

Simplify the logic to clarify this.

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>