devlink: Change netlink attribute validation

Currently the netlink attribute resolving is done by a sequence of
if's. Change the attribute resolving to table lookup.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>