devlink: fix flexible_array.cocci warning

Fix following coccicheck warning:
./net/core/devlink.c:69:6-10: WARNING use flexible-array member instead

Signed-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Link: https://lore.kernel.org/r/20211103121607.27490-1-guozhengkui@vivo.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>