net: Fix oops in dev_ifsioc()

A command like this: "brctl addif br1 eth1" issued as a user gave me
an oops when bridge module wasn't loaded. It's caused by using a dev
pointer before checking for NULL.

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>