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: 5028588b62cb ("ethtool: wire up set policies to ops")
Reported-by: Leon Romanovsky <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>
Tested-by: Leon Romanovsky <[e-mail]>
Reviewed-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ethtool/netlink.c

···
 		.cmd	= ETHTOOL_MSG_CHANNELS_SET,
 		.flags	= GENL_UNS_ADMIN_PERM,
 		.doit	= ethnl_set_channels,
-		.policy = ethnl_channels_get_policy,
-		.maxattr = ARRAY_SIZE(ethnl_channels_get_policy) - 1,
+		.policy = ethnl_channels_set_policy,
+		.maxattr = ARRAY_SIZE(ethnl_channels_set_policy) - 1,
 	},
 	{
 		.cmd	= ETHTOOL_MSG_COALESCE_GET,