hinic: fix sending pkts from core while self testing

Call netif_tx_disable firstly before starting doing self-test to
avoid sending packet from networking core and self-test packet
simultaneously which may cause self-test failure or hw abnormal.

Fixes: 4aa218a4

 ("hinic: add self test support")
Signed-off-by: Luo bin <luobin9@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>