mac80211: protect ieee80211_assign_beacon with next_beacon check

Even if it is not a real issue since ieee80211_set_after_csa_beacon()
or ieee80211_set_after_color_change_beacon() are run only when csa or bcc
is active, move next_beacon check before running ieee80211_assign_beacon
routine.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/041764ed7e9781bcee66c33b41f1365aa4205932.1649327683.git.lorenzo@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>