media: v4l: event: Add subscription to list before calling "add" operation

Patch ad608fbc changed how events were subscribed to address an issue
elsewhere. As a side effect of that change, the "add" callback was called
before the event subscription was added to the list of subscribed events,
causing the first event queued by the add callback (and possibly other
events arriving soon afterwards) to be lost.

Fix this by adding the subscription to the list before calling the "add"
callback, and clean up afterwards if that fails.

Fixes: ad608fbc ("media: v4l: event: Prevent freeing event subscriptions while accessed")

Reported-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: stable@vger.kernel.org (for 4.14 and up)
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>