powerpc/pseries: Use the security flags in pseries_setup_rfi_flush()

powerpc/pseries: Use the security flags in pseries_setup_rfi_flush()

BugLink: https://bugs.launchpad.net/bugs/1760099
CVE-2017-5754

Now that we have the security flags we can simplify the code in
pseries_setup_rfi_flush() because the security flags have pessimistic
defaults.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 2e4a16161fcd324b1f9bf6cb6856529f7eaf0689 linux-next)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>