gpiolib: keep comment consistent with code

The commit f881bab0 ("gpio: keep the GPIO line names internal")
change the error to warning in gpiochip_set_desc_names. Update the
comment accordingly.

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>