pcmcia: db1xxx_ss: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings (Building: db1xxx_defconfig mips):

drivers/pcmcia/db1xxx_ss.c:257:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/pcmcia/db1xxx_ss.c:269:3: warning: this statement may fall through [-Wimplicit-fallthrough=]

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>