net: hns3: add support to set/get rx buf len via ethtool for hns3 driver

Rx buf len is for rx BD buffer size, support setting it via ethtool -G
parameter and getting it via ethtool -g parameter.

Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>