stmhal: Fix USB_VCP.recv so that it returns actual amount of bytes read.

Addresses issue #1529.

stmhal/usb.c

@@ -440,6 +440,7 @@ STATIC mp_obj_t pyb_usb_vcp_recv(mp_uint_t n_args, const mp_obj_t *args, mp_map_
     if (o_ret != MP_OBJ_NULL) {
         return mp_obj_new_int(ret); // number of bytes read into given buffer
     } else {
+        vstr.len = ret; // set actual number of bytes read
         return mp_obj_new_str_from_vstr(&mp_type_bytes, &vstr); // create a new buffer
     }
 }