mac80211: fix user-power when emulating chanctx

ieee80211_calc_hw_conf_chan was ignoring the configured
user_txpower.  If it is set, use it to potentially decrease
txpower as requested.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Link: https://patch.msgid.link/20241010203954.1219686-1-greearb@candelatech.com


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