iwlagn: update beacon smarter

Updating the beacon every time right after one was
transmitted is pointless, most of the time we might
not even have to update it. We will update it every
time it changes, which includes from set_tim(), a
callback iwlwifi didn't implement so far.

This also reduces latency for clients, previously
we would update the beacon right after the previous
one was transmitted, and then a TIM change would
only take effect after that again -- updating the
beacon right after the TIM changes makes the TIM
change go out to the air faster.

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