iwlwifi: mvm: Loosen the channel context/phy context coupling

In current implementation, the phy context is tightly coupled
with the channel context. This complicates the possibility of
using the same phy context for both netdev interfaces and the P2P
Device interface. To loosen this coupling:

1. Manage all the phy contexts in the mvm op mode, and only save
   the phy context id in the chanctx memory.
2. Reference count the phy contexts and free them only when they
   are not longer used (both by mac80211 and P2P Device).

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>