net: ethtool: fix the error condition in ethtool_get_phy_stats_ethtool()

In the Linux kernel, the following vulnerability has been resolved: net: ethtool: fix the error condition in ethtool_get_phy_stats_ethtool() Clang static checker (scan-build) warning: net/ethtool/ioctl.c:line 2233, column 2 Called function pointer is null (null dereference). Return '-EOPNOTSUPP' when 'ops->get_ethtool_phy_stats' is NULL to fix this typo error.