net: lwip: Use ipaddr helpers

net: lwip: Use ipaddr helpers

The ip_addr_t of lwIP has support for both IPv6 and IPv4 addresses.
Some lwIP commans is directly accessing the internal addr field of the
ip_addr_t instead of using ipaddr helper functions.

Change to use ipaddr helper functions where appropriate to remove direct
access of the internal addr field. Also change a few instances from ip4
to the version less ipaddr helpers.

There is no intended functional change, besides the change from using
ip4 addr helper to using version less ipaddr helper.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Jerome Forissier <jerome.forissier@arm.com>