ipv6: fix checkpatch errors in net/ipv6/addrconf.c

ERROR: code indent should use tabs where possible: fix 2.
ERROR: do not use assignment in if condition: fix 5.

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>