fix raw_socket on macos

[hikaricai]

Currently BfpDevice::interface_mtu() return 4096, and RawSocket::new() would add it with EthernetFrame::header_len(), so BfpDevice::recv() would return "Invalid argument (os error 22)".
This patch avoids adding mtu with ether_hdr_len for raw_socket on macos, maybe we will also avoid adding Ieee802154 extral hdr_len.

[whitequark]

I wonder if we should have a macOS test runner?

[thvdveld]

smoltcp currently doesn't have any device implementation for macOS, I think (correct me if I'm wrong)? The TUN/TAP devices are only compiled for Linux or Android. Is the BfpDevice something we can add to smoltcp? Or could you share where I could find this implementation?

[thvdveld]

@whitequark I made changes in my fork to add macOS test runners (thvdveld#8), however, I'm not sure if it is really necessary. I don't think smoltcp contains code that is specific for macOS. As explained in my previous comment, smoltcp does not have a macOS specific device, such as the tuntap devices for Linux.

Let me know if I need to open a PR to add the macOS runners.