kaweth: fix firmware download

commit 60bcabd080f53561efa9288be45c128feda1a8bb upstream.

This fixes the oops discovered by the Umap2 project and Alan Stern.
The intf member needs to be set before the firmware is downloaded.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>