Documentation: gpio: legacy: Don't use POLLERR for poll(2)

According to the manpage of poll(2) and also looking at the respective
syscall providing POLLERR in .events is a no-op. So don't recommend
using it.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>