arm-smmu: Fix incorrect memset argument order

The order of arguments to memset is destination, value and then
size.

Change-Id: I159a6ff42f6d8117fc17f7a3802a19d06d53d3c1
Cc: Patrick Daly <pdaly@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>