net: hns: Fix ethtool private flags

The driver implementation returns support for private flags, while
no private flags are present. When asked for the number of private
flags it returns the number of statistic flag names.

Fix this by returning EOPNOTSUPP for not implemented ethtool flags.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>