gpiolib: Split GPIO flags parsing and GPIO configuration

When requesting a GPIO through the legacy or the gpiod_* API the
gpiochip request operation is first called and then the GPIO flags are
parsed and the GPIO is configured. This prevents the gpiochip from
rejecting the request if the flags are not supported by the device.

To fix this split the parse-and-configure operation in two and parse
flags before requesting the GPIO.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>