After the Host has read
a message or a sequence of messages from Rx FIFO 1 it has to write the buffer index of
the last element read from Rx FIFO 1 to F1AI. This will set the Rx FIFO 1 Get Index
RXF1S.F0GI to F1AI + 1 and update the FIFO 1 Fill Level
RXF1S.F1FL.