brcmsmac: AP mode: update beacon when TIM changes

brcmsmac: AP mode: update beacon when TIM changes

Beacons are not updated to reflect TIM changes. This is not compliant with
power-saving client stations as the beacons do not have valid TIM and can
cause the network to stall at random occasions and to have highly variable
latencies.
Fix it by updating beacon templates on mac80211 set_tim callback.

Addresses an issue described in:
https://marc.info/?i=20180911163534.21312d08%20()%20manjaro

Signed-off-by: Ali MJ Al-Nasrawy <alimjalnasrawy@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>