drivers: net: declance: fix comparing pointer to 0

Fixes coccicheck warning:

./drivers/net/ethernet/amd/declance.c:611:14-15:
	WARNING comparing pointer to 0

Replace "skb == 0" with "!skb".

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>