mgmt: mcumgr: Fix remaining net API use

In d45cd6716bbab3a805e3a5fd461934f0dcdc13e5 the mayority of the
Zephyr codebased was changed to use the Zephyr native net_ prefixed
types, but some were forgotten.
Without this fix/change the code still builds as we are by now setting
CONFIG_NET_NAMESPACE_COMPAT_MODE. But when this is not set, things
fail to build.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>