soc: rockchip: io-domain: Fix error return code in rockchip_iodomain_probe()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: e943c43b32ce ("PM: AVS: rockchip-io: Move the driver to the rockchip specific drivers")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Zhang Changzhong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Heiko Stuebner <[e-mail]>

drivers/soc/rockchip/io-domain.c

···
 		if (uV < 0) {
 			dev_err(iod->dev, "Can't determine voltage: %s\n",
 				supply_name);
+			ret = uV;
 			goto unreg_notify;
 		}