Fix the AR724x PCIe glue to correctly probe the BAR on AR7240 devices.

There's a bug in the AR7240 PCIe hardware where a correct BAR will end
up having the device disappear.

It turns out that for the device address it should be all 0's.

However, this meant that the PCI probe code would try writing 0xffffffff
in to see how big the window was, read back 0x0, and think the window
was 32 bits.  It then ended up calculating a resource size of 0 bytes,
failed to find anything via an rman call, and this would fail to attach.

I have quite absolutely no idea how in the various planes of existence
this particular bit of code and how it worked with the PCI bus code
ever worked.  But, well, it did.

Tested:

* Atheros AP93 - AR7240 + AR9280 reference board
Convert to if_get_counter().
Provide ixgb_get_counter().