wifi: mac80211: correctly initialise S1G chandef for STA

wifi: mac80211: correctly initialise S1G chandef for STA

When moving to the APs channel, ensure we correctly initialise the chandef
and perform the required validation. Additionally, if the AP is beaconing on a
2MHz primary, calculate the 2MHz primary center frequency by extracting
the sibling 1MHz primary and averaging the frequencies to find the 2MHz
primary center frequency.

Signed-off-by: Lachlan Hodges <lachlan.hodges@morsemicro.com>
Link: https://patch.msgid.link/20250918051913.500781-3-lachlan.hodges@morsemicro.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>