net/hns3: fix mailbox response length

net/hns3: fix mailbox response length

Remove the macro 'HNS3_REG_MSG_DATA_OFFSET' which is used to prevent
access violation for response array by limiting the response data length
to 4.
But the limit value is too short to get some longer information such as
6 byte MAC address.

This patch modifies the length of response data from mailbox to allow
the response data length to be 8. So that the VF driver could get more
data from PF driver by mailbox.

Fixes: 463e748964f5 ("net/hns3: support mailbox")
Cc: stable@dpdk.org
Signed-off-by: Chengchang Tang <tangchengchang@hisilicon.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>