iwlwifi: mvm: don't keep an mvm ref when the interface is down

There is no reason to keep a reference when the interface is down,
since we are not really doing anything.  The reference is only needed
when the mac80211 start op (or a hw restart) is running, to prevent
going into runtime or system supend in the meantime.  This will allow
us to support runtime PM when the interface is down (in another
patch).

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>