pinctrl: amd: avoid maybe-uninitalized warning

Since gpio_dev->hwbank_num is now a variable, the compiler cannot
figure out if pin_num is initialized at all:

drivers/pinctrl/pinctrl-amd.c: In function 'amd_gpio_dbg_show':
drivers/pinctrl/pinctrl-amd.c:210:3: warning: 'pin_num' may be used uninitialized in this function [-Wmaybe-uninitialized]
   for (; i < pin_num; i++) {
   ^~~
drivers/pinctrl/pinctrl-amd.c:172:21: warning: 'i' may be used uninitialized in this function [-Wmaybe-uninitialized]

This adds a 'default' statement to make that case well-defined.

Fixes: 3bfd4430 ("pinctrl: amd: Add support for additional GPIO")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>