ethtool: rss: fix rss key initialization warning

ethtool: rss: fix rss key initialization warning

This warning is emitted when a driver does not default populate an rss
key when one is not provided from userspace. Some devices do not
support individual rss keys per context. For these devices, it is ok
to leave the key zeroed out in ethtool_rxfh_context. Do not warn on
zeroed key when ethtool_ops.rxfh_per_ctx_key == 0.

Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20241003162310.1310576-1-daniel.zahka@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>