net: hns: Adjust the SBM module buffer threshold

HNS needs SMB Buffers to store at least two packets after sending
pause frame because of the link delay. The MTU of HNS is 9728. As
the processor user manual described, the SBM buffer threshold should
be modified.

Reported-by: Ping Zhang <zhangping5@huawei.com>
Signed-off-by: Kejian Yan <yankejian@huawei.com>
Reviewed-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>