iwlwifi: mvm: sanity check context in iwl_mvm_phy_ctxt_unref()

In some botched (!) restart scenarios we seem to get here with
a NULL PHY context, so warn and exit instead of crashing.

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