MAINTAINERS: Update mailing list and web page for hwmon subsystem

The old web page for the hwmon subsystem is no longer operational,
and the mailing list has become unreliable. Move both to kernel.org.
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>