wifi: mac80211: support initialising current S1G short beacon index

wifi: mac80211: support initialising current S1G short beacon index

Introduce the sb_count variable which tracks the number of
beacon intervals until the next long beacon. To initialise this
value, we find the current short beacon index into this period
which represents the number of short beacons left to send before
the next long beacon. We use the same TSF value used to initialise
the DTIM count to ensure the short beacon count and DTIM count
are in sync as its common for the long beacon period and DTIM period
to be equivalent.

Signed-off-by: Lachlan Hodges <lachlan.hodges@morsemicro.com>
Link: https://patch.msgid.link/20250717074205.312577-4-lachlan.hodges@morsemicro.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>