pinctrl: rockchip: Mark expected switch fall-through

When fall-through warnings was enabled by default the following warning
was starting to show up:

../drivers/pinctrl/pinctrl-rockchip.c: In function ‘rockchip_gpio_set_config’:
../drivers/pinctrl/pinctrl-rockchip.c:2783:3: warning: this statement may fall
 through [-Wimplicit-fallthrough=]
   rockchip_gpio_set_debounce(gc, offset, true);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../drivers/pinctrl/pinctrl-rockchip.c:2795:2: note: here
  default:
  ^~~~~~~

Rework so that the compiler doesn't warn about fall-through. Add
'return -ENOTSUPP;' to match the comment.

Fixes: d93512ef0f0e ("Makefile: Globally enable fall-through warning")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Link: https://lore.kernel.org/r/20190726112812.19665-1-anders.roxell@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>