kernel/queue: Fix SMP race

Calling z_ready_thread() means the thread is now ready and can wake up
at any moment on another CPU.  But we weren't finished setting the
return value!  So the other side could wake up with a spurious "error"
condition if it ran too soon.  Note that on systems with a working
IPI, that wakeup can happen much faster than you might think.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>