gpio: sysfs: repair export returning -EPERM on 1st attempt

It would make sense to return -EPERM if the bit was already set (already
used), not if it was cleared. Before this fix pins can only be exported on
the 2nd attempt:

$ echo 522 > /sys/class/gpio/export
sh: write error: Operation not permitted
$ echo 522 > /sys/class/gpio/export

Fixes: 35b5453 ("gpio: remove gpio_lock")
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpiolib-sysfs.c

@@ -593,7 +593,7 @@ int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
 	if (!guard.gc)
 		return -ENODEV;
 
-	if (!test_and_set_bit(FLAG_EXPORT, &desc->flags))
+	if (test_and_set_bit(FLAG_EXPORT, &desc->flags))
 		return -EPERM;
 
 	gdev = desc->gdev;