iwlwifi: mvm: beautify code in BT Coex

The iterators don't need to know what bt_kill_msk means.
All they need to know is if reduced Tx power is enabled
on an interface or not. So change the member of the
iterator to be a bool.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>