gpio: sysfs: remove redundant export tests

The attribute operations will never be called for an unregistered device
so remove redundant checks for FLAG_EXPORT.

Note that kernfs will also guarantee that any active sysfs operation has
finished before the attribute is removed during deregistration.

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>