net: feature-gate nightly-only async traits to allow building on stable.

examples/stm32l5/src/bin/usb_ethernet.rs

@@ -12,14 +12,13 @@ use embassy_net::tcp::TcpSocket;
 use embassy_net::{PacketBox, PacketBoxExt, PacketBuf, Stack, StackResources};
 use embassy_stm32::rcc::*;
 use embassy_stm32::rng::Rng;
-use embassy_stm32::time::Hertz;
 use embassy_stm32::usb::Driver;
 use embassy_stm32::{interrupt, Config};
 use embassy_sync::blocking_mutex::raw::ThreadModeRawMutex;
 use embassy_sync::channel::Channel;
 use embassy_usb::{Builder, UsbDevice};
 use embassy_usb_ncm::{CdcNcmClass, Receiver, Sender, State};
-use embedded_io::asynch::{Read, Write};
+use embedded_io::asynch::Write;
 use rand_core::RngCore;
 use static_cell::StaticCell;
 use {defmt_rtt as _, panic_probe as _};