net: caif: check return value of alloc_netdev

I don't know if dev can actually be NULL here, but the test should be
above alloc_netdev(), to avoid leaking the struct net_device in case
dev is actually NULL. And of course the return value from alloc_netdev
should be tested.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>