net: hns3: change the method of obtaining default ptp cycle

The ptp cycle is related to the hardware, so it may cause compatibility
issues if a fixed value is used in driver. Therefore, the method of
obtaining this value is changed to read from the register rather than
use a fixed value in driver.

Fixes: 0bf5eb78 ("net: hns3: add support for PTP")
Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>