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

examples/rp/src/bin/usb_ethernet.rs

@@ -16,7 +16,7 @@ use embassy_sync::blocking_mutex::raw::ThreadModeRawMutex;
 use embassy_sync::channel::Channel;
 use embassy_usb::{Builder, Config, UsbDevice};
 use embassy_usb_ncm::{CdcNcmClass, Receiver, Sender, State};
-use embedded_io::asynch::{Read, Write};
+use embedded_io::asynch::Write;
 use static_cell::StaticCell;
 use {defmt_rtt as _, panic_probe as _};