After the Host has read a message or a sequence of messages from Rx
FIFO 0 it has to write the buffer index of the last element read from Rx FIFO 0 to F0AI.
This will set the Rx FIFO 0 Get Index RXF0S.F0GI to F0AI + 1 and update the FIFO 0 Fill
Level RXF0S.F0FL.