gpio: bail out silently on NULL descriptors

gpio: bail out silently on NULL descriptors

In 2c8a7b4b85c5e398877213f539d928895938f3fc
("gpio: reflect base and ngpio into gpio_device")
assumed that GPIO descriptors are either valid or error
pointers, but gpiod_get_[index_]optional() actually return
NULL descriptors and then all subsequent calls should just
bail out.

Cc: stable@vger.kernel.org
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Fixes: 2c8a7b4b85c5 ("gpio: reflect base and ngpio into gpio_device")
Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>