leds: gpio: Fix semantic error

The leds-gpio driver mixes up the legacy GPIO flags with the
GPIO descriptor flags and passes a legacy flag to
devm_gpiod_get_index().

Fix this by replacing the flags variable with the strict
descriptor flag.

Fixes: 45d4c6de ("leds: gpio: Try to lookup gpiod from device")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>