mach-k3: common: correct the calculations for determining firewalls

The background firewall calculations were wrong, fix that to determine
both the background and foreground correctly.

Fixes: 8bfce2f9 ("arm: mach-k3: common: reorder removal of firewalls")

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>