pinctrl: nuvoton: wpcm450: fix out of bounds write

Write into 'pctrl->gpio_bank' happens before the check for GPIO index
validity, so out of bounds write may happen.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: a1d1e0e3

 ("pinctrl: nuvoton: Add driver for WPCM450")
Signed-off-by: Mikhail Kobuk <m.kobuk@ispras.ru>
Reviewed-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20230825101532.6624-1-m.kobuk@ispras.ru
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>