media: ov5640: fix support of BT656 bus mode

Fix PCLK polarity not being taken into account.
Add comments about BT656 register control.
Remove useless ov5640_set_stream_bt656() function.
Refine comments about MIPI IO register control.

Fixes: 4039b037 ("media: i2c: ov5640: Add support for BT656 mode")
Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>