net: hns3: change SSU's buffer allocation according to UM

Currently when there is share buffer in the SSU(storage
switching unit), the low waterline for RX private buffer is
too low to keep the hardware running. Hardware may have
processed all the packet stored in the private buffer of the
low waterline before the new packet comes, because hardware
only tell the peer send packet again when the private buffer
is under the low waterline.

So this patch only allocate RX private buffer if there is
enough buffer according to hardware user manual.

This patch also reserve some buffer for reusing when TC num
is less than or equal to 2, and change PAUSE_TRANS_GAP &
HCLGE_NON_DCB_ADDITIONAL_BUF according to hardware user
manual.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>