MIPS: fix some more fall through errors in arch/mips

Fix these errors:

 arch/mips/cavium-octeon/executive/cvmx-pko.c:489:7: error: this statement may fall through [-Werror=implicit-fallthrough=]
 arch/mips/bcm63xx/dev-flash.c:89:3: error: this statement may fall through [-Werror=implicit-fallthrough=]
 arch/mips/ath79/setup.c:155:17: error: this statement may fall through [-Werror=implicit-fallthrough=]
 arch/mips/ar7/setup.c:50:3: error: this statement may fall through [-Werror=implicit-fallthrough=]

Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Crispin <john@phrozen.org>
Cc: Matthias Schiffer <mschiffer@universe-factory.net>
Cc: "Petr Štetiar" <ynezz@true.cz>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Burton <paul.burton@mips.com>