Bluetooth: mesh: Do not set RFU bits in Heartbeat Publication Features

Features received in Config Heartbeat Publication Set message can have
Feature bits set to RFU values.
This patch fixes setting this RFU bits in Heartbeat Publication
Features, so that those are not indicated in Config Heartbeat
Publication Status message.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>