swap set/reset priority for DOI

Continuation of #187 (closed).

APB is access OK, but the problem would still occur on AXI/Avalon (or whichever bus which has one-cycle transactions). As a solution it would be enough to swap set/reset priority for DOI. The SW/HW race condition which it would introduce is still here, so nothing is lost.

Tasks:

• change implementation
• modify tests
• modify documentation