rpl_udp: Addresses must only be 8-bit with cc110x

Most of our devices, especially in the IEEE 802.15.4 world, use 16-bit short addresses, instead of 8-bit.
This PR allows `rpl_udp` to use the preconfigured short address of `auto_init`, regardless of there value space.

As a bonus `auto_init` configures addresses in the 8-bit space if the application uses a cc110x device.
As you're improving things here anyways - why don't you move the test further up - it seems odd to check if the command should be run only at the end of the function ;).