k_fifo/K_lifo macros: avoid unnecessary casts

The fifo/lifo API is implemented on top of the queue API with macros
that blindly force a cast to struct k_queue. Providing a reference to
the _queue member from the k_fifo structure is much cleaner as it let
the compiler perform pointer type checking. Generated code is identical.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>