drivers: mailbox: rpmh: Correct matching of tcs command sequence

Fix sanity check to match new commands with existing sequence.
Also add check for selected slot to be less than maximum available
slots.

Change-Id: Ib3fc3d87d53d3ac47e53119b0cd4b998d19656ba
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>