pinctrl: bcm2835: Use string_choices API instead of ternary operator

Use modern string_choices API instead of manually determining the
output using ternary operator.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20240603181938.76047-3-wahrenst@gmx.net


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>