ring_buffer: fix designated initializer order in RING_BUF_DECLARE

Change order of designated initializers for ring_buf struct in the
RING_BUF_DECLARE macro, to match the order in the struct definition.
This fixes compilation when using C++, which requires using the same
order as in the struct definition.
Fixes #45697

Signed-off-by: Jonas Otto <jonas@jonasotto.com>