arm64: KVM: hyp: debug-sr: Mark expected switch fall-through

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

../arch/arm64/kvm/hyp/debug-sr.c: In function ‘__debug_save_state’:
../arch/arm64/kvm/hyp/debug-sr.c:20:19: warning: this statement may fall
 through [-Wimplicit-fallthrough=]
  case 15: ptr[15] = read_debug(reg, 15);   \
../arch/arm64/kvm/hyp/debug-sr.c:113:2: note: in expansion of macro ‘save_debug’
  save_debug(dbg->dbg_bcr, dbgbcr, brps);
  ^~~~~~~~~~
../arch/arm64/kvm/hyp/debug-sr.c:21:2: note: here
  case 14: ptr[14] = read_debug(reg, 14);   \
  ^~~~
../arch/arm64/kvm/hyp/debug-sr.c:113:2: note: in expansion of macro ‘save_debug’
  save_debug(dbg->dbg_bcr, dbgbcr, brps);
  ^~~~~~~~~~
../arch/arm64/kvm/hyp/debug-sr.c:21:19: warning: this statement may fall
 through [-Wimplicit-fallthrough=]
  case 14: ptr[14] = read_debug(reg, 14);   \
../arch/arm64/kvm/hyp/debug-sr.c:113:2: note: in expansion of macro ‘save_debug’
  save_debug(dbg->dbg_bcr, dbgbcr, brps);
  ^~~~~~~~~~
../arch/arm64/kvm/hyp/debug-sr.c:22:2: note: here
  case 13: ptr[13] = read_debug(reg, 13);   \
  ^~~~
../arch/arm64/kvm/hyp/debug-sr.c:113:2: note: in expansion of macro ‘save_debug’
  save_debug(dbg->dbg_bcr, dbgbcr, brps);
  ^~~~~~~~~~

Rework to add a 'Fall through' comment where the compiler warned
about fall-through, hence silencing the warning.

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