freebsd_network: Adjust and rewrite attach code for USB support.

init_hardware now takes 2 devices arrays: one for PCI and one for USB
drivers, respectively.

The few drivers that have their own __haiku_handle_fbsd_drivers_list
are also adjusted at the same time.

drivers/network: Move ethernet drivers to an "ether" subdirectory.

Now that there is also a "tap" driver in here, and the WiFi drivers
are also in a subdirectory, moving the physical ethernet device drivers
to a subdirectory also makes sense.

(Briefly) discussed on the mailing lists.