iwlwifi: mvm: schedule CSA time event a bit before beacon 1

Instead of using a hardcoded number of TUs before beacon 0 as the time
to start the absence and actual channel switch, calculate it in
relation to the beacon interval.  We use 10 TUs + beacon interval
before beacon 0 to target a bit before beacon 1.  This gives us enough
time to switch to the new channel before the AP/GO switches.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>