If you read about the 16 byte FIFO operation on any of the SERCOM modules (USART, SPI, I2C) all of them states that you should enable the FIFO by writing CTRLC.FIFOEN = '1'.
Problem is that the CTRLC register is not described in the Register Summary or Register Descriptor section for any of the SERCOMs.
Also if you search the sercom.h CMSIS header files no CTRLC definitions exists...
So what happended to the FIFO? I really liked the idea since it heavily optimized the sercom operation.