net: cavium: liquidio: use helpers ns_to_timespec64()

Convert the driver to use ns_to_timespec64() to keep consistency
with timespec64_to_ns() instead of open coding the same logic.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>