ethtool: fix error paths in ethnl_set_channels()

Fix two error paths in ethnl_set_channels() to avoid lock-up caused
but unreleased RTNL.

Fixes: e19c591e ("ethtool: set device channel counts with CHANNELS_SET request")
Reported-by: LiLiang <liali@redhat.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Link: https://lore.kernel.org/r/20201215090810.801777-1-ivecera@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>