void z8530_flush_fifo (struct z8530_channel * c);
Flush the receive FIFO. There is no specific option for this, we blindly read bytes and discard them. Reading when there is no data is harmless. The 8530 has a 4 byte FIFO, the 85230 has 8 bytes.
All locking is handled for the caller. On return data may still be present if it arrived during the flush.