mac80211_hwsim: call ieee80211_tx_prepare_skb under RCU protection

mac80211_hwsim: call ieee80211_tx_prepare_skb under RCU protection

This is needed since it might use (and pass out) pointers to
e.g. keys protected by RCU. Can't really happen here as the
frames aren't encrypted, but we need to still adhere to the
rules.

Fixes: 420d7a13af50 ("mac80211_hwsim: initialize ieee80211_tx_info at hw_scan_work")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20220505230421.5f139f9de173.I77ae111a28f7c0e9fd1ebcee7f39dbec5c606770@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>