net: hns3: Fix for PF mailbox receving unknown message

Before the firmware updates the crq's tail pointer, if the PF driver
reads the data in the crq, the data may be incomplete at this time,
which will lead to the driver read an unknown message.

This patch fixes it by checking if crq is not empty before reading the
message.

Fixes: c1a81619 ("net: hns3: Add mailbox interrupt handling to PF driver")
Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>