wifi: mac80211: fix idle calculation with multi-link

The vif's idle state doesn't automatically go to true when
any link removes the channel context, it's only idle when
_all_ links no longer have a channel context. Fix that.

Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240418105220.90df97557702.I05d2228ce85c203b9f2d6da8538cc16dce46752a@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>