wifi: mac80211_hwsim: use hrtimer_active()

Instead of hrtimer_is_queued(), use hrtimer_active() since
it might be running concurrently, and then it's not queued
at that point in time, as suggested by Thomas Gleixner in
https://lore.kernel.org/87plqn5psu.ffs@tglx

, I just never
got to this for ages.

I think the concurrency is otherwise fine since we'll get
to cancel if we're actually removing things, and otherwise
we just send a beacon at slightly the wrong time or so.

Reported-by:  <syzbot+41e4341f493f1155aa3d@syzkaller.appspotmail.com>
Link: https://patch.msgid.link/20241011145230.5a4d38d4ff9b.Iac0ec316a0c9a7b2619abe52ddc8e04c25d8c7e1@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>