firewire: Provide module aliase for backwards compatibility.

This patch loads fw-sbp2 if sbp2 is still in the config file. So one can
go back and forth between releases without worry about the root
filesystem drivers.

Signed-off-by: Kristian Hoegsberg <krh@redhat.com>

Existing mkinitrd scripts still have to be adapted, unless they grok
module aliases.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>