iwlwifi: mvm: Change PHY context handling

1. All the phy contexts are added immediately after the
   firmware is loaded and up.
2. Whenever a PHY context needs to be used, its reference
   counter is incremented and the PHY context is being
   configured to the appropriate configuration.
3. When a PHY context is no longer needed, its reference
   count is decremented.
4. PHY contexts are never removed.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>