Bluetooth: Mesh: Regenerate private beacon even when disabled

Even if PRB state is disabled, PRB can still be sent over GATT if Proxy
Privacy parameter is enabled. In such case PRB won't be regenerated.
To keep the privacy of the network, PRB should be regenerated when sent
over GATT regardless of PRB state.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>