stmhal: Stop USB before entering DFU by software.

stmhal/modpyb.c

@@ -65,6 +65,7 @@
 /// \function bootloader()
 /// Activate the bootloader without BOOT* pins.
 STATIC NORETURN mp_obj_t pyb_bootloader(void) {
+    pyb_usb_dev_stop();
     storage_flush();
 
     HAL_RCC_DeInit();

stmhal/usb.c

@@ -75,6 +75,13 @@ void pyb_usb_dev_init(usb_device_mode_t mode, usb_storage_medium_t medium) {
 #endif
 }
 
+void pyb_usb_dev_stop(void) {
+    if (dev_is_enabled) {
+        USBD_Stop(&hUSBDDevice);
+        dev_is_enabled = 0;
+    }
+}
+
 bool usb_vcp_is_enabled(void) {
     return dev_is_enabled;
 }

stmhal/usb.h

@@ -42,6 +42,7 @@ typedef enum {
 } usb_storage_medium_t;
 
 void pyb_usb_dev_init(usb_device_mode_t mode, usb_storage_medium_t medium);
+void pyb_usb_dev_stop(void);
 bool usb_vcp_is_enabled(void);
 bool usb_vcp_is_connected(void);
 void usb_vcp_set_interrupt_char(int c);