pinctrl: amd: fix compilation warning

3bfd4430 ("pinctrl: amd: Add support for additional GPIO")
created the following warning:

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]
  unsigned int bank, i, pin_num;
                     ^
Fix this by adding a guarding default case for illegal
bank numbers.

Cc: S-k Shyam-sundar <Shyam-sundar.S-k@amd.com>
Cc: Nehal Shah <Nehal-bakulchandra.Shah@amd.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>