fix(drivers/tzc400): never disable filter 0

Disabling filter 0 causes inability to access DRAM.
An attempt leads to an abort.
ARM manual disallows to disable filter 0, but if we do
that from SRAM, nothing bad happens.

This patch prevents disabling of a filter 0, allowing to
reconfigure other filters from DRAM.

Note: this patch doesn't change the logic after reset.
It is only needed in case someone wants to reconfigure the
previously configured TZASC.

Change-Id: I196a0cb110a89afbde97f64a94df3101f28708a4
1 file changed