media: rcar-csi2: Serialize access to set_fmt and get_fmt

The access to the internal storage of the format rcar_csi2.mf should be
serialized, extend the existing lock mutex to also cover this.

While at it document the mutex.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>