drivers: rtio: bugfix

fixes the following:
- If a read SQE generates a CQE with an error before a buffer
has been allocated,
the flags of the sqe are not correctly translated into cqe flags.
- rtio_cqe_get_mempool_buffer(...) sets the *buff
to a non-zero value, even if no buffer is assigned

Signed-off-by: Florian Weber <Florian.Weber@live.de>