tuntap: fix error return code in tun_set_iff()

Fix to return a negative error code from the error handling
case instead of 0, as returned elsewhere in this function.

[ Bug added in linux-3.8 , commit 4008e97f
  ("tuntap: fix ambigious multiqueue API") ]

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>