fix(el3-spmc): fix incomplete reclaim validation

Ensure that the full memory transaction descriptor has been transmitted
before a request to reclaim the memory transaction is permitted.
This prevents any potential accesses to the incomplete descriptor.
Reported by Matt Oh, Google Android Red Team.

Reported-by:  <mattoh@google.com>
Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I33e993c6b754632051e899ab20edd19b18b6cf65