To harden the event queue mechanism, we add a running index to the
control header of the entry.

The firmware writes the index in each entry and the driver verifies
that the index of the current entry is larger by 1 of the index of
the previous entry.

In case it isn't, the driver will treat the entry as if it wasn't
valid (it won't process it but won't skip it).
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>