cpu/cc26xx_cc13xx: fix typo on XOSC selection
cpu/cc26xx_cc13xx: fix typo on XOSC selection

> Fixes a typo on XOSC selection bitfield that would
make the CPU crash when changing it.
> Sets the other fields to their default values.

Signed-off-by: Jean Pierre Dudey <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@jeandudey.tech>

OTAkeys/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,miri64/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,authmillenon/RIOT