wifi: cfg80211: use structs for TBTT information access

Make the data access a bit nicer overall by using structs. There is a
small change here to also accept a TBTT information length of eight
bytes as we do not require the 20 MHz PSD information.

This also fixes a bug reading the short SSID on big endian machines.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230618214436.4c3f8901c1bc.Ic3e94fd6e1bccff7948a252ad3bb87e322690a17@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>