gpio: aspeed: fix a potential NULL pointer dereference

In case devm_kzalloc, the patch returns ENOMEM to avoid potential
NULL pointer dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>