wifi: mac80211: fix min center freq offset tracing

We need to set the correct trace variable, otherwise we're
overwriting something else instead and the right one that
we print later is not initialized.

Fixes: b6011960

 ("mac80211: handle channel frequency offset")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230504134511.828474-2-gregory.greenman@intel.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>