Force user to initialize HSI48 before using USB

This prevents mistakes (USB won't work without HSI48) and restores the
behavior of the old API, before the recent changes.

Author: hannobraun

examples/usb_serial.rs

@@ -16,11 +16,11 @@ fn main() -> ! {
 
     let mut rcc = dp.RCC.freeze(rcc::Config::hsi16());
     let mut syscfg = SYSCFG::new(dp.SYSCFG, &mut rcc);
-    rcc.enable_hsi48(&mut syscfg, dp.CRS);
+    let hsi48 = rcc.enable_hsi48(&mut syscfg, dp.CRS);
 
     let gpioa = dp.GPIOA.split(&mut rcc);
 
-    let usb = USB::new(dp.USB, gpioa.pa11, gpioa.pa12);
+    let usb = USB::new(dp.USB, gpioa.pa11, gpioa.pa12, hsi48);
     let usb_bus = UsbBus::new(usb);
 
     let mut serial = SerialPort::new(&usb_bus);

src/usb.rs

@@ -12,7 +12,10 @@
 //! Please check out the USB examples in the `examples/` directory to see how it
 //! fits together.
 
-use crate::pac::{self, RCC};
+use crate::{
+    pac::{self, RCC},
+    rcc::HSI48,
+};
 use stm32_usbd::UsbPeripheral;
 
 use crate::gpio::gpioa::{PA11, PA12};
@@ -27,6 +30,7 @@ impl USB {
         _:   pac::USB,
         _dm: PA11<Input<Floating>>,
         _dp: PA12<Input<Floating>>,
+        _:   HSI48,
     )
         -> Self
     {