More progress towards getting xmit to work. At the moment I'm seeing
PCIe errors when alx attaches, so I'm doing something wrong. Need to fix
that first before working more on the transmit stuff.
Transmit is working! Turned out that I had to tell the hardware about
the addresses of the rx rings too.

This change adds link state change handling and fixes a bunch of bugs.
I'm still seeing the PCIe error, so that's going to be the next thing to
tackle.
Get rid of all the commented-out code from the Linux driver.
Add a bunch of debugging in an attempt to track down these PCIe errors.
No luck yet.