Clear the indirect flag in the GICv3 ITS driver

Summary:
The indirect flag tells the hardware to use a flat or two level table.
As we only support using the flat table ensure the flag that marks
which is in use is set correctly.

We can't rely on this being set correctly as some firmware may set the
indirect flag, e.g. booting from LinuxBoot.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36873