Move USB network drivers to drivers/net/usb.

It is preferable to group drivers by usage (net, scsi, ATA, ...) than
by bus.  When reviewing drivers, the [PCI|USB|PCMCIA|...] maintainer
is probably less qualified on networking issues than a networking
maintainer.  Also, from a practical standpoint, chips often
appear on multiple buses, which is why we do not put drivers into
drivers/pci/net.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>