UBUNTU: Add asix to nic-usb-modules file

BugLink: http://bugs.launchpad.net/bugs/499785

Modern USB NIC adapters (USB2.0 / Gb networking) mostly use the asix
chipset. In order to support them in the installer, we need to have the
asix minimodule enabled.  Add this to the d-i nic-usb-modules udeb.

Signed-off-by: Leann Ogasawara <leann.ogasawara@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>