The SSTP and DSTP bits determine whether or not to disable the hardware triggers
(SIRQEN = 0), once a DMA message has completed.
When the SSTP bit is set and the DMAnSCNT = 0, then the
SIRQEN bit will be cleared. Similarly, when the DSTP bit is set and the DMAnDCNT =
0, the SIRQEN bit will be cleared.