net: net_if: fix net_if_send_data for offloaded ifaces

Some offloaded ifaces have an L2, but lack support for
net_l2->send. This edge case is not handled by
net_if_send_data, resulting in a NULL dereference under
rare circumstances.

This patch expands the offloaded iface guard in
net_if_send_data to handle this edge case.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>