extmod/vfs: Raise OSError(ENODEV) if mounting bdev without a filesystem.

This commit prevents uos.mount() from raising an AttributeError.
vfs_autodetect() is supposed to return an object that has a "mount" method,
so if no filesystem is found it should raise an OSError(ENODEV) and not
return the bdev itself which has no "mount" method.

extmod/vfs.c

@@ -191,7 +191,8 @@ STATIC mp_obj_t mp_vfs_autodetect(mp_obj_t bdev_obj) {
     return mp_fat_vfs_type.make_new(&mp_fat_vfs_type, 1, 0, &bdev_obj);
     #endif
 
-    return bdev_obj;
+    // no filesystem found
+    mp_raise_OSError(MP_ENODEV);
 }
 
 mp_obj_t mp_vfs_mount(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {