drivers: i2s: stm32: use length from i2s_write

The length parameter from `i2s_write` is not used correctly by
i2s_ll_stm32.c. E.g. `i2s_write(dev, block, 0)` would work.

Use the length from `i2s_write` in `dma_reload` to fix the issue.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>