PREV_TX
#define PREV_TX(i) (((i) - 1) & (SEEQ_TX_BUFFERS - 1))
#define PREV_TX(num) (((num) - 1) & (TX_RING_SIZE - 1))
#define PREV_TX(num) (((num) - 1) & (TX_RING_SIZE - 1))
#define PREV_TX(num) (((num) - 1) & (TX_RING_MAXSIZE - 1))