media: staging: media: imx: Update TODO entry

Update the TODO entry that mentioned a potential use case for the now
removed g_mbus_config video operation.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>