wifi: mac80211: add and use chanctx usage iteration

wifi: mac80211: add and use chanctx usage iteration

In preparation for NAN interfaces using multiple channel
contexts, add an iterator macro that iterates all users
of a given channel context.

The logic during reserved assign/reassign handling the
bandwidth in ieee80211_get_chanctx_max_required_bw() is
a bit more complicated and should be cleaned up, so it
isn't yet converted.

Link: https://patch.msgid.link/20251105160431.5aaccc2f127d.I2b7fd0858a263916f43abab49c6d3cc0b5aa16ec@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>