8139too: Add netif_napi_del in the driver

For linux-3.18.0
The driver lacks netif_napi_del in the normal path and error path
to match the call of netif_napi_add in rtl8139_init_one.
This patch fixes this problem.

Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>