powerpc/powernv: Fix fall-through warning for Clang

Fix the following fallthrough warnings (powernv_defconfig and powerpc64):

drivers/char/powernv-op-panel.c:78:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>