pinctrl: bcm2835: Fix permissions of persist_gpio_outputs

pinctrl: bcm2835: Fix permissions of persist_gpio_outputs

The commit 8ff05989b44e ("pinctrl: bcm2835: Make pin freeing behavior
configurable") unintentionally made the module parameter
persist_gpio_outputs changeable at runtime. So drop the write permission
in order to make the freeing behavior predictable for user applications.

Fixes: 8ff05989b44e ("pinctrl: bcm2835: Make pin freeing behavior configurable")
Reported-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Closes: https://lore.kernel.org/linux-gpio/Zjk-C0nLmlynqLAE@surfacebook.localdomain/
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20240603181938.76047-2-wahrenst@gmx.net
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>