SPI FIFO Status Register
| rf_cnt | RXFIFO Counter These bits indicate the number of bytes in RXFIFO |
| rb_cnt | RXFIFO Write Buffer Counter |
| rb_wr | RXFIFO Write Buffer Write Enable |
| tf_cnt | TXFIFO Counter These bits indicate the number of bytes in TXFIFO |
| tb_cnt | TXFIFO Write Buffer Counter |
| tb_wr | TXFIFO Write Buffer Write Enable |