[IPV6]: Cleanup snmp6_alloc_dev()

This functions is never called with NULL or not setup argument,
so the checks inside are redundant.

Also, the return value is always -ENOMEM, so no need in
additional variable for this.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>