Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: bcm63xx_defconfig mips):

arch/mips/pci/ops-bcm63xx.c: In function ‘bcm63xx_pcie_can_access’:
arch/mips/pci/ops-bcm63xx.c:474:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (PCI_SLOT(devfn) == 0)
      ^
arch/mips/pci/ops-bcm63xx.c:477:2: note: here
  default:
  ^~~~~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: linux-mips@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org