RX FIFO status
| USED | Number of entries in the RX FIFO. The field value is in the range [0, 256]. |
| RD_PTR | RX FIFO read pointer: FIFO location from which a data frame is read by the host. This field is used only for debugging purposes. |
| WR_PTR | RX FIFO write pointer: FIFO location at which a new data frame is written by the hardware. This field is used only for debugging purposes. |