soc: qcom: smem: Ensure ordering of the allocated entry

The remote cpus might not use locking when looking for smem entries, so
it's important that the header is fully prepared before we mark it as
allocated.

Also, we can drop the wmb() right before we unlock the hwspinlock as
this contains a memory barrier.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>