gpiolib: of_find_gpio(): Don't discard errors

Since commit dd34c37a ("gpio: of: Allow -gpio suffix for property
names") when requesting a GPIO from the devicetree gpiolib looks for
properties with both the '-gpio' and the '-gpios' suffix. This was
implemented by first searching for the property with the '-gpios' suffix
and if that yields an error try the '-gpio' suffix. This approach has the
issue that any error returned when looking for the '-gpios' suffix is
silently discarded.

Commit 06fc3b70 ("gpio: of: Fix handling for deferred probe for -gpio
suffix") partially addressed the issue by treating the EPROBE_DEFER error
as a special condition. This fixed the case when the property is specified,
but the GPIO provider is not ready yet. But there are other cases in which
of_get_named_gpiod_flags() returns an error even though the property is
specified, e.g. if the specification is incorrect.

of_find_gpio() should only try to look for the property with the '-gpio'
suffix if no property with the '-gpios' suffix was found. If the property
was not found of_get_named_gpiod_flags() will return -ENOENT, so update the
condition to abort and propagate the error to the caller in all other
cases.

This is important for gpiod_get_optinal() and friends to behave correctly
in case the specifier contains errors. Without this patch they'll return
NULL if the property uses the '-gpios' suffix and the specifier contains
errors, which falsely indicates to the caller that no GPIO was specified.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>