media: platform: renesas-ceu: drop buf NULL check

Since start_streaming is only called if there are at least two
buffers queued, the ceudev->capture list will never be empty, so
the check whether there are no buffers can be dropped.

Note that the '!buf' check was wrong in any case, if we wanted to
check for an empty list it should have used list_empty().

This fixes this smatch warning:

drivers/media/platform/renesas/renesas-ceu.c:705 ceu_start_streaming() warn: can 'buf' even be NULL?

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>