uart: Fix thinko in output ring pointers

I accidentally typed uint8_t instead of uint32_t... thanks Joel.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
uart: Works better if we actually allocate out_buf

That got lost during a rebase ... oops.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>