devlink: don't work with possible NULL pointer in devlink_param_unregister()

There is a WARN_ON checking the param_item for being NULL when the param
is not inserted in the list. That indicates a driver BUG. Instead of
continuing to work with NULL pointer with its consequences, return.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>