wifi: ieee80211: Fix the common size calculation for reconfiguration ML

The common information length is found in the first octet of the common
information.

Fixes: 0f48b8b8 ("wifi: ieee80211: add definitions for multi-link element")
Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230618214435.3c7ed4817338.I42ef706cb827b4dade6e4ffbb6e7f341eaccd398@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>