devlink: remove devlink_dump_for_each_instance_get() helper

devlink_dump_for_each_instance_get() is currently called from
a single place in netlink.c. As there is no need to use
this helper anywhere else in the future, remove it and
call devlinks_xa_find_get() directly from while loop
in devlink_nl_instance_iter_dump(). Also remove redundant
idx clear on loop end as it is already done
in devlink_nl_instance_iter_dump().

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