media: rcar-vin: remove unneeded locking in async callbacks
The locking implemented in the async notifier callbacks are unnecessary
as the global list_lock in v4l2-async.c is held whenever one of the
callbacks are called.
The locking in itself is not harmful however it produces a LOCKDEP
warning between the global v4l2-async list_lock and the rcar-vin local
locking schema. Remove the rcar-vin locking for the async callbacks to
reduce complexity and silent the false LOCKDEP warning.