[DMA] use FIRQ select instead of FIRQ mask

[stnolting]

⚠️ The "auto-configuration" of the DMA now uses a 4-bit FIRQ select field (to select only a single FIRQ trigger) instead of a 16-bit FIRQ mask (potentially selecting more than one FIRQ trigger).

Selecting more than one trigger does not make much sense. Furthermore, the trigger logic would no longer react when more than one FIRQ channel is enabled while one of the triggers keeps pending (effectively ignoring the second one).