media: rkisp1: capture: Fix and simplify (un)registration

The rkisp1_register_capture() and rkisp1_unregister_capture() functions
don't destroy the mutex (in the error path for the former). Fix this and
make rkisp1_unregister_capture() and rkisp1_capture_devs_unregister()
safe to be called on an unregistered capture node to prepare for
simplification of error handling at probe time.

While at it, drop the double initialization of cap->rkisp1 in
rkisp1_capture_devs_register() as the field is already initialized in
rkisp1_capture_init().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>