net: z85230: replace comparison to NULL with "!skb"

According to the chackpatch.pl, comparison to NULL could
be written "!skb".

Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>