media: ipu3-cio2: Serialise access to pad format

Pad format can be accessed from user space. Serialise access to it.

Fixes: c2a6a07a

 ("media: intel-ipu3: cio2: add new MIPI-CSI2 driver")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Bingbu Cao <bingbu.cao@intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: stable@vger.kernel.org # v4.16 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>