iwlwifi: rename iwl_mac_beacon_update

Rename iwl_mac_beacon_update to iwlcore_beacon_update
and make the calling convention a bit different. The
old name with _mac_ indicated that it was a mac80211
callback, but that's no longer true.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>