ethtool: correct policy for ETHTOOL_MSG_CHANNELS_SET

This accidentally got wired up to the *get* policy instead
of the *set* policy, causing operations to be rejected. Fix
it by wiring up the correct policy instead.

Fixes: 5028588b ("ethtool: wire up set policies to ops")
Reported-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Tested-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>