net: hns3: Fix for mailbox message truncated problem

The payload of mailbox message is 16 byte and the value of
HCLGE_MBX_MAX_ARQ_MSG_SIZE is 8. A message truncated problem will
happen when mailbox message is converted to ARQ message. This patch
replaces HCLGE_MBX_MAX_ARQ_MSG_SIZE with the size of ARQ message in
hclgevf_mbx_handler to fix this problem.

Fixes: b11a0bb2 ("net: hns3: Add mailbox support to VF driver")
Signed-off-by: Fuyun Liang <liangfuyun1@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>