media: stih-cec: add missing io.h

In the driver the io.h is implied by others. This is not good as it
prevents from cleanups done in other headers. Add missing include.

Link: https://lkml.kernel.org/r/20240423192529.3249134-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Alain Volmat <alain.volmat@foss.st.com>
Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Samuel Holland <samuel@sholland.org>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: Sean Young <sean@mess.org>
Cc: Stefani Seibold <stefani@seibold.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>