ipip: fix sparse warnings in ipip_netlink_parms()

This change fixes two sparse warnings triggered by casting the ip addresses
from netlink messages in an u32 instead of be32. This change corrects that
in order to resolve the sparse warnings.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>