dmaengine: ste_dma40: don't allow high priority dest event lines
Hardware bug: when a logical channel is triggerred by a high priority
destination event line, an extra packet transaction is generated in case
of important data write response latency on previous logical channel A
and if the source transfer of current logical channel B is already
completed and if no other channel with a higher priority than B is
waiting for execution.
Software workaround: do not set the high priority level for the
destination event lines that trigger logical channels.