dpll: check that pin is registered in __dpll_pin_unregister()

Similar to what is done in dpll_device_unregister(), add assertion to
__dpll_pin_unregister() to make sure driver does not try to unregister
non-registered pin.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Arkadiusz Kubalewski <arkadiusz.kubalewski@intel.com>
Link: https://lore.kernel.org/r/20240206074853.345744-1-jiri@resnulli.us


Signed-off-by: Jakub Kicinski <kuba@kernel.org>