docs: hwmon: Use file modes explicitly

As stated in the mainling list[1] and by checkpatch output, the file modes
for is_visible_func should be returned explicitly. Change that in the
documentation to avoid confusion.

[1]https://lore.kernel.org/linux-hwmon/506a6e7f-4566-2dcf-37f3-0f41f4ce983b@roeck-us.net/



Signed-off-by: Joaquín Ignacio Aramendía <samsagax@gmail.com>
Link: https://lore.kernel.org/r/20221212141245.52935-1-samsagax@gmail.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>