ACPICA: vsnprintf: this statement may fall through

[rjw:] Add an explicit switch () statement fall-through marker.

Signed-off-by: Toomas Soome <tsoome@me.com>
Signed-off-by: Erik Schmauss <erik.schmauss@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>