mac80211_hwsim: fix RCU protected chanctx access

We need to RCU protect the chanctx_conf access, so
do that.

Fixes: 585625c9

 ("mac80211_hwsim: check TX and STA bandwidth")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20220505230421.fb8055c081a2.Ic6da3307c77a909bd61a0ea25dc2a4b08fe1b03f@changeid


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