The Data Scramble Control (DSCC) must be configured before the CTRLA.ENABLE is set. These settings cannot be changed while the module is enabled.
The scrambling logic is enabled by writing one to the enable bit in the Data Scramble Control register (DSCC.DSCEN). Scrambling is disabled by writing a zero to DSCC.DSCEN. Writing a zero to CTRLA.ENABLE will also disable the scrambling, but will not clear the DSCC.DSCEN bit.