drivers: i2s: use new k_mem_slab definition

Commit 2f003e59 ("kernel: Re-factor k_mem_slab definition") moved
block_size into from k_mem_slab to k_mem_slab_info without updating i2s
handlers. Use the new member to fix build failures.

Fixes: #63363

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>