gpiolib: remove unneeded WARN_ON() from gpiochip_set_multiple()

gpiolib: remove unneeded WARN_ON() from gpiochip_set_multiple()

GPIO drivers are not required to support set_multiple() - the core will
fallback to calling set() for each line if it's missing. Remove the
offending check from gpiochip_set_multiple().

Fixes: 98ce1eb1fd87 ("gpiolib: introduce gpio_chip setters that return values")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Closes: https://lore.kernel.org/all/ab3e42c0-70fa-48e0-ac93-ecbffef63507@samsung.com/
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250227152831.59784-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>