hwmon: tmp401: Read "ti,n-factor" as signed

hwmon: tmp401: Read "ti,n-factor" as signed

The "ti,n-factor" binding and examples allow negative correction
values. Reading it as u32 makes the helper type disagree with the
documented signed value and hides real schema mismatches.

Use the signed helper so the DT access matches the s32 value stored by
the driver.

Assisted-by: Codex:gpt-5-5
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20260612215332.1889497-1-robh@kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>