gpio: adp5520: cleanup probe error path + remove platform_set_drvdata()

The platform_set_drvdata() call is only useful if we need to retrieve back
the private information.
Since the driver doesn't do that, it's not useful to have it.

This also means that the 'err' label can be removed and all goto statements
replaced with direct returns (with error codes).

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>