devlink: Make some functions static

Fixes the following sparse warnings:

net/core/devlink.c:2297:25: warning:
 symbol 'devlink_resource_find' was not declared. Should it be static?
net/core/devlink.c:2322:6: warning:
 symbol 'devlink_resource_validate_children' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>