net: fix mistake path for netdev_features_strings

Th_strings arrays netdev_features_strings, tunable_strings, and
phy_tunable_strings has been moved to file net/ethtool/common.c.
So fixes the comment.

Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>