media: platform: stm32: unprepare clocks at handling errors in probe

stm32_cec_probe() did not unprepare clocks on error handling paths. The
patch fixes that.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Evgeny Novikov <novikov@ispras.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>