gpio: aggregator: Fix gpio_aggregator_line_alloc() checking

gpio: aggregator: Fix gpio_aggregator_line_alloc() checking

The gpio_aggregator_line_alloc() function returns error pointers, but
the callers check for NULL.  Update the error checking in the callers.

Fixes: 83c8e3df642f ("gpio: aggregator: expose aggregator created via legacy sysfs to configfs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Koichiro Den <koichiro.den@canonical.com>
Link: https://lore.kernel.org/r/cc71d8cf6e9bb4bb8cd9ae5050100081891d9345.1744452787.git.dan.carpenter@linaro.org
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>