hwmon: (scmi) return -EINVAL when sensor information is unavailable

Passing NULL pointer to PTR_ERR will result in return value of 0
indicating success which is clearly not what it is intended here.

This patch returns -EINVAL instead when the sensor information is not
available.

Fixes: b23688ae

 ("hwmon: add support for sensors exported via ARM SCMI")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Cc: linux-hwmon@vger.kernel.org
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>