media: rcar-vin: Improve async notifier cleanup paths

The cleanup code for the async notifiers can be refactored to own
functions to reduce code duplication and improve readability. While at
it rename the CSI-2 initialization function _csi2_ instead of _mc_ to
match.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>