iwlwifi: mvm: rename iwl_mvm_new_bt_coex_notif

It actually handles a BT coex notification, so rename it
to be more self explained.
Also, this function can always look at mvm->last_bt_notif
provided that the latter is updated on time.

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