Don't overwrite /etc/default/rng-tools

The init script (`/etc/init.d/rng-tools`) auto-detects the appropriate device and `/dev/hwrng` is the first and the default entry in its search list, so no need to explicitly set it here.

Also, not overwriting `/etc/default/rng-tools` enables users to set rngd's options in `RNGDOPTIONS`.

See xbianonpi/xbian#534.