media: mtk-vcodec: fix access to vb2_v4l2_buffer struct

Commit 0650a914 ("media: mtk-vcodec: Correct return type for mem2mem
buffer helpers") fixed the return types for mem2mem buffer helper
functions, but omitted two occurrences that are accessed in the
mtk_v4l2_debug() macro. These only trigger compiler errors when DEBUG is
defined.

Fixes: 0650a914 ("media: mtk-vcodec: Correct return type for mem2mem buffer helpers")

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>