sorry, was just updating (realised my checkout of embassy was old).
these changes were not enough for embassy to work:

```
diff --git a/examples/stm32f4/src/bin/usb_serial.rs b/examples/stm32f4/src/bin/usb_serial.rs
index f3a375d31..b852717af 100644
--- a/examples/stm32f4/src/bin/usb_serial.rs
+++ b/examples/stm32f4/src/bin/usb_serial.rs
@@ -13,7 +13,7 @@ use futures::future::join;
 use {defmt_rtt as _, panic_probe as _};
 
 bind_interrupts!(struct Irqs {
-    OTG_FS => usb::InterruptHandler<peripherals::USB_OTG_FS>;
+    OTG_HS => usb::InterruptHandler<peripherals::USB_OTG_HS>;
 });
 
 #[embassy_executor::main]
@@ -25,7 +25,7 @@ async fn main(_spawner: Spawner) {
         use embassy_stm32::rcc::*;
         config.rcc.hse = Some(Hse {
             freq: Hertz(8_000_000),
-            mode: HseMode::Bypass,
+            mode: HseMode::Oscillator,
         });
         config.rcc.pll_src = PllSource::HSE;
         config.rcc.pll = Some(Pll {
@@ -47,7 +47,7 @@ async fn main(_spawner: Spawner) {
     let mut ep_out_buffer = [0u8; 256];
     let mut config = embassy_stm32::usb::Config::default();
     config.vbus_detection = true;
-    let driver = Driver::new_fs(p.USB_OTG_FS, Irqs, p.PA12, p.PA11, &mut ep_out_buffer, config);
+    let driver = Driver::new_fs(p.USB_OTG_HS, Irqs, p.PB15, p.PB14, &mut ep_out_buffer, config);
 
     // Create embassy-usb Config
     let mut config = embassy_usb::Config::new(0xc0de, 0xcafe);
```