Add some comments about the TXWI fields and the TX_STA_FIFO register.
Especially describe the relationship between the TXWI field PACKETID
and the PID field in the TX_STA_FIFO register.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>