rtio: Clear flags in prep helpers

Tests were failing when the sqe was allocated on the stack for some
architectures. Initialization of variables on the stack is not guaranteed
to be zeroed like static data. This caused undefined behavior.

Secondly if the sqe is recycled and had a flag set, there would
unexpected behavior as well.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>