rtio: Fix unused argument

When compiling with CONFIG_RTIO_SYS_MEM_BLOCKS=n, rtio.h causes unused
argument warnings. Align the implementation of rtio_block_pool_free with
rtio_block_pool_alloc and add ARG_UNUSED() statements for the arguments.

Signed-off-by: Marco Widmer <marco.widmer@bytesatwork.ch>