rtio: workq: bugfix of memory allocation

This commit fixes the bug that the memory of the work request is
freed up in the work handler, before it is not needed anymore by the p4wq.
This is fixed now, by using the new done_handler in the p4wq
for freeing up that memory.

Signed-off-by: Florian Weber <Florian.Weber@live.de>
(cherry picked from commit b55b9ae7)