media: stm32-dcmi: clarify state logic on buffer starvation

Introduce WAIT_FOR_BUFFER state instead of "active" field checking
to manage buffer starvation case.

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