hwmon: (core) Clarify when read and write callbacks are mandatory

hwmon: (core) Clarify when read and write callbacks are mandatory

The callback descrption in hwmon.h was misleading and stated that read and
write callbacks would be optional. More accurate is is that the callbacks
are mandatory if readable / writeable attributes are present.

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