stm32/usb: Expose USB HID device instance via usbd_hid_get().

This is needed to implement a HID device in user C modules.

ports/stm32/usb.c

@@ -367,6 +367,24 @@ usbd_cdc_itf_t *usb_vcp_get(int idx) {
     return &usb_device.usbd_cdc_itf[idx];
 }
 
+#if MICROPY_HW_USB_HID
+
+// return hid interface if hid is configured, NULL otherwise
+usbd_hid_itf_t *usbd_hid_get(void) {
+    #if defined(USE_HOST_MODE)
+    return NULL;
+    #else
+    uint8_t usb_mode = USBD_GetMode(&usb_device.usbd_cdc_msc_hid_state) & USBD_MODE_IFACE_MASK;
+    if (usb_mode == USBD_MODE_HID || usb_mode == USBD_MODE_CDC_HID || usb_mode == USBD_MODE_MSC_HID) {
+        return &usb_device.usbd_hid_itf;
+    } else {
+        return NULL;
+    }
+    #endif
+}
+
+#endif
+
 /******************************************************************************/
 // MicroPython bindings for USB
 

ports/stm32/usbd_hid_interface.h

@@ -16,6 +16,8 @@ typedef struct _usbd_hid_itf_t {
     uint8_t report_in_buf[HID_DATA_FS_MAX_PACKET_SIZE];
 } usbd_hid_itf_t;
 
+usbd_hid_itf_t *usbd_hid_get(void);
+
 static inline int usbd_hid_rx_num(usbd_hid_itf_t *hid) {
     return hid->report_in_len != USBD_HID_REPORT_INVALID;
 }