platform/x86: panasonic-laptop: Return errno correctly in show callback

When an error occurs in sysfs show callback, we should return the errno
directly instead of formatting it as the result, which produces
meaningless output and doesn't inform the userspace of the error.

Fixes: 468f96bf ("platform/x86: panasonic-laptop: Add support for battery charging threshold (eco mode)")
Fixes: d5a81d8e ("platform/x86: panasonic-laptop: Add support for optical driver power in Y and W series")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20241118064637.61832-3-ziyao@disroot.org


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>