net: dsa: bcm_sf2: 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>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>