MIPS: Annotate fall-through in Cavium Octeon code

There's an intentional switch case fall-through in Cavium Octeon USB
code, which triggers compile errors with -Wimplicit-fallthrough due to
-Werror being enabled for arch/mips.

This can be encountered when building cavium_octeon_defconfig.

Fix the build issue by annotating the intentional fall-through.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org