devlink: put couple of WARN_ONs in devlink_param_driverinit_value_get()

Put couple of WARN_ONs in devlink_param_driverinit_value_get() function
to clearly indicate, that it is a driver bug if used without reload
support or for non-driverinit param.

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>