PCI: Fix fall-through warning for Clang

Fix the following fallthrough warning (arm64-randconfig with Clang):

drivers/pci/proc.c:234:3: warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/


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