Use contructor to create USB peripheral

This follows the standards established by other peripheral APIs, and
prevents the user from messing with the USB peripheral or the pins after
initialization.

Author: hannobraun

examples/usb_serial.rs

@@ -20,11 +20,7 @@ fn main() -> ! {
 
     let gpioa = dp.GPIOA.split(&mut rcc);
 
-    let usb = USB {
-        usb: dp.USB,
-        pin_dm: gpioa.pa11,
-        pin_dp: gpioa.pa12,
-    };
+    let usb = USB::new(dp.USB, gpioa.pa11, gpioa.pa12);
     let usb_bus = UsbBus::new(usb);
 
     let mut serial = SerialPort::new(&usb_bus);

src/usb.rs

@@ -19,10 +19,19 @@ use crate::gpio::gpioa::{PA11, PA12};
 use crate::gpio::{Floating, Input};
 pub use stm32_usbd::UsbBus;
 
-pub struct USB {
-    pub usb: pac::USB,
-    pub pin_dm: PA11<Input<Floating>>,
-    pub pin_dp: PA12<Input<Floating>>,
+
+pub struct USB(());
+
+impl USB {
+    pub fn new(
+        _:   pac::USB,
+        _dm: PA11<Input<Floating>>,
+        _dp: PA12<Input<Floating>>,
+    )
+        -> Self
+    {
+        Self(())
+    }
 }
 
 unsafe impl Sync for USB {}