crypto: sun4i-ss - simplify optional reset handling

As of commit bb475230 ("reset: make optional functions really
optional"), the reset framework API calls use NULL pointers to describe
optional, non-present reset controls.

This allows to unconditionally return errors from
devm_reset_control_get_optional_exclusive.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>