mtd: nand: sunxi: 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 return errors from devm_reset_control_get_optional and to
call reset_control_(de)assert unconditionally.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>