drivers/net/acenic.c: fix check-after-use

The Coverity checker noted that we've already dereferenced "dev" when we
check whether it's NULL.

Since it's impossible that "dev" is NULL at this place this patch
removes the NULL check.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>