staging: comedi: ni_labpc: cleanup trigger setting in labpc_ai_cmd()

The (*do_cmdtest) already validated the cmd sources so the default
cases in the switch() statements can never happen.

Preclear to trigger bits and change the switch() statements to simple
if() tests to set the bits.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>