mac80211_hwsim: hrtimer beacon

For testing various timing-sensitive protocols (power
save, MBCA, etc.), a beacon accuracy of jiffies is not
sufficient. A tasklet_hrtimer is used for the beacon since
it runs the callback in soft-IRQ context with hrtimer
resolution.

Also handle BSS_CHANGED_BEACON_ENABLED for hwsim.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
[simplify timer container_of]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>