KVM: arm: vgic-v3: Mark expected switch fall-through

When fall-through warnings was enabled by default the following warnings
was starting to show up:

../virt/kvm/arm/hyp/vgic-v3-sr.c: In function ‘__vgic_v3_save_aprs’:
../virt/kvm/arm/hyp/vgic-v3-sr.c:351:24: warning: this statement may fall
 through [-Wimplicit-fallthrough=]
   cpu_if->vgic_ap0r[2] = __vgic_v3_read_ap0rn(2);
   ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
../virt/kvm/arm/hyp/vgic-v3-sr.c:352:2: note: here
  case 6:
  ^~~~
../virt/kvm/arm/hyp/vgic-v3-sr.c:353:24: warning: this statement may fall
 through [-Wimplicit-fallthrough=]
   cpu_if->vgic_ap0r[1] = __vgic_v3_read_ap0rn(1);
   ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
../virt/kvm/arm/hyp/vgic-v3-sr.c:354:2: note: here
  default:
  ^~~~~~~

Rework so that the compiler doesn't warn about fall-through.

Fixes: d93512ef0f0e ("Makefile: Globally enable fall-through warning")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>