sierra_net: Add support for IPv6 and Dual-Stack Link Sense Indications

If a context is configured as dualstack ("IPv4v6"), the modem indicates
the context activation with a slightly different indication message.
The dual-stack indication omits the link_type (IPv4/v6) and adds
additional address fields.
IPv6 LSIs are identical to IPv4 LSIs, but have a different link type.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>