hwmon: (nct6775) Fix fan speed set failure in automatic mode

hwmon: (nct6775) Fix fan speed set failure in automatic mode

[ Upstream commit 8b3800256abad20e91c2698607f9b28591407b19 ]

Setting the fan speed is only valid in manual mode; it is not possible
to set the fan's speed in automatic mode.
Return error when attempting to set the fan speed in automatic mode.

Signed-off-by: Xing Tong Wu <xingtong.wu@siemens.com>
Link: https://lore.kernel.org/r/20231121081604.2499-3-xingtong_wu@163.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>