[BugFix][FFI] Add a missing default for datatype lanes (apache#16649)

A default value for lanes was unintentionally removed in apache#16612, this
PR fixes this which in turn fixes the test failure seen in
`test_tensor_dtype_lanes` in CI.

python/tvm/_ffi/runtime_ctypes.py

@@ -140,6 +140,8 @@ def __init__(self, type_str):
             self.lanes = ctypes.c_uint16(-int(arr[2]))
         elif len(arr) > 1:
             self.lanes = ctypes.c_uint16(int(arr[1]))
+        else:
+            self.lanes = 1
         bits = 32
 
         if head.startswith("int"):