ring_buffer: rationalize the item-based put and get methods

Conceptually, ring_buf_item_put() and ring_buf_item_get() are specialized
versions of ring_buf_put() and ring_buf_get(). Make it so to rationalize
the code to open the way for more optimizations.

This means we need specialized wrappers on top of ring_buf_init()
accordingly, given that the core machinery is now common and byte based.

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