tests: rtio: Fix potential race in iodev test

Race was possible though very unlikely between the atomic cas
and queue push/pop operations. The outcome of the race had it shown up
would have been a submission not worked on due to the timer never being
started. A small critical section fixes this and clarifies where the single
conumer part of the mpsc queue comes in despite there being multiple
contexts which may enter that section.

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