dmaengine: at_xdmac: only monitor overflow errors for peripheral xfer
The overflow error flag (ROI: Request Overflow Error) is only relevant
for the case when the channel handles a peripheral synchronized transfer.
Not in the case of memory to memory transfer where there is no hardware
request signal.
Remove the use of this interrupt source in such a case. It's based on
the first descriptor which holds the configuration for the whole
linked list transfer.