wifi: mac80211: clarify per-STA bandwidth handling

wifi: mac80211: clarify per-STA bandwidth handling

The per-STA bandwidth handling is completely confusing, given
the function names etc.

Move everything to sta_info.c and rename the functions to
accurately reflect what they return:
 - ieee80211_sta_bw_capability()
 - ieee80211_sta_current_bw() can return the appropriate
   bandwidth in the desired direction (a new enum)

At any given time, the bandwidth with which we expect to receive
frames from a station may differ from the bandwidth with which
we may transmit frames to the station; this will happen either
during RX OMI negotiation, or for a long time if the station
used an HT Notify Channel Width frame. We also implement the
(VHT) Operating Mode Notification as an asymmetric setting, even
if the spec would seem to imply it could be symmetric.

Also rename the 'cur_max_bandwidth' value to 'op_mode_bw' which
matches the 'op_mode_nss', indicating more clearly what it _is_
rather than how it's _used_. It's not quite precise (NSS is)
since it's also HT chanwidth notification, but that seems OK.

Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20260415144514.09ae71b74d5c.Ib59c2ac82e030559d1f7d462f06ba3488a090946@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>