It seems that enabling Tx and Rx before setting descriptor DMA

It seems that enabling Tx and Rx before setting descriptor DMA
addresses shall access invalid descriptor DMA addresses on PCIe
hardwares and then panicked the system.
To fix it set descriptor DMA addresses before enabling Tx and Rx
such that hardware can see valid descriptor DMA addresses. Also
set RL_EARLY_TX_THRESH before starting Tx and Rx.

Reported by:	steve.tell AT crashmail DOT de
Tested by:	steve.tell AT crashmail DOT de
Obtained from:	NetBSD
MFC after:	1 week