pinctrl: nsp-gpio: forever loop in nsp_gpio_get_strength()

There is a signedness bug here so the loop will never exit.

Fixes: 8bfcbbbc ('pinctrl: nsp: add gpio-a driver support for Broadcom NSP SoC')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>