media: platform: stm32: wait end of transmission

It is mandatory to write CEC_CFGR only when CECEN=0. To protect
transmission, a check have been added to delayed logical address
modification. This patch is necessary tp pass all tests of compliance.

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>