asix-sigma: track whether triggers were specified when acquisition started

asix-sigma: track whether triggers were specified when acquisition started

There are several separate conditions which the driver needs to tell
apart. There is a compile time switch whether trigger support shall be
built in. There is the condition whether acquisition start involved a
user provided trigger spec. And there is the hardware flag whether a
previously configured trigger condition matched and where its position
is.

Only accept user provided trigger specs when trigger support is builtin.
(The get/set/list availability and spec passing is done in applications
outside of the library, we better check just to make sure.) Only setup
the trigger related hardware parameters when a spec was provided. Only
check for trigger positions when the hardware detected a match.