iwlwifi: trans: turn set_pmi into an optional callback

It is not currently implemented for SDIO, and not required for other slave
buses as well.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>

drivers/net/wireless/iwlwifi/iwl-trans.h

@@ -787,7 +787,8 @@ static inline u32 iwl_trans_write_mem32(struct iwl_trans *trans, u32 addr,
 
 static inline void iwl_trans_set_pmi(struct iwl_trans *trans, bool state)
 {
-	trans->ops->set_pmi(trans, state);
+	if (trans->ops->set_pmi)
+		trans->ops->set_pmi(trans, state);
 }
 
 static inline void