net: hns3: aligning buffer size in SSU to 256 bytes

The hardware expects the buffer size set to SSU is aligned to
256 bytes, this patch aligns the buffer size to 256 byte using
roundup or rounddown function.

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