Tethering: Skip link local addresses when enabling NAT

Only one entry is allowed to be added to route table 60 for all
interface ipv6 link local addresses, so skip ipv6 link local
addresses on all interfaces when enabling NAT

Change-Id: Ibbe6c2bdae83586cdd38287089715f3433bbb91f
Signed-off-by: jiaguo <jiaguo@marvell.com>