media: i2c: adv748x: Store the pixel rate ctrl on CSI objects

The current implementation has to search the list of controls for the
pixel rate control, each time it is set.  This can be optimised easily
by storing the ctrl pointer in the CSI/TX object, and referencing that
directly.

While at it, fix up a missing blank line also highlighted in review
comments.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>