usb: legacy: Fix net API use

In c52c206e26f90df85eceafd7dbbebaae5066c753 this code was changed to use
the Zephyr native net_ prefixed API, but some were forgotten
(ntohs()). Let's change it now.

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>