scripts/coccinelle/free: add NULL test before dev_{put, hold} functions

Since commit b37a4668

 ("netdevice: add the case if dev is NULL"),
NULL check before dev_{put, hold} functions is not needed.

Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>