media: stm32-dcmi: fix crash when subdev do not expose any formats

Do not access sd_formats[] if num_of_sd_formats is zero, ie
subdev sensor didn't expose any formats.

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>