iwlwifi: lower BT coex aggregation message severity

As the rate scaling algorithm will attempt to enable
aggregation over and over again, the message will
flood the log if there is, for example, Bluetooth
streaming music. Make it a debug messages instead of
printing it all the time.

Reported-by: Jan-Michael Brummer <jan.brummer@tabos.org>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>