USB iowarrior.c: fix check-after-use

The Coverity checker spotted that we have already oops'ed if "dev"
was NULL.

Since "dev" being NULL doesn't seem to be possible here this patch 
removes the NULL check.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>