vendorcode/amd/agesa: f15tn & f16kb: fix assert

Fixes warning by GCC 7.1:
note: did you mean to use logical not?

Change-Id: If8167c6fe88135ae89eb795eeda09e6937b1684f
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20698
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
2 files changed