gpio: bail out silently on NULL descriptors

gpio: bail out silently on NULL descriptors

In fdeb8e1547cb9dd39d5d7223b33f3565cf86c28e
("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: [email protected]
Cc: Sergei Shtylyov <[email protected]>
Cc: Florian Fainelli <[email protected]>
Cc: Andrew Lunn <[email protected]>
Fixes: fdeb8e1547cb ("gpio: reflect base and ngpio into gpio_device")
Reported-by: Uwe Kleine-König <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>