hwmon: (gl520sm) De-macro the sysfs callbacks

Use standard dynamic sysfs callbacks instead of macro-generated
wrappers. This makes the code more readable, and the binary smaller
(by about 11%).
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>