iwlwifi: always check if got h/w access before write

Before we write to the device registers always check if
iwl_grap_nic_access() was successful.

On the way change return type of grab_nic_access() to bool, and add
likely()/unlikely() statement.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-agn.c b/drivers/net/wireless/iwlwifi/iwl-agn.c
index 5f390f8..ce9ebeb 100644
--- a/drivers/net/wireless/iwlwifi/iwl-agn.c
+++ b/drivers/net/wireless/iwlwifi/iwl-agn.c
@@ -327,7 +327,7 @@
 
 	/* Make sure device is powered up for SRAM reads */
 	spin_lock_irqsave(&trans(priv)->reg_lock, reg_flags);
-	if (iwl_grab_nic_access(trans(priv))) {
+	if (unlikely(!iwl_grab_nic_access(trans(priv)))) {
 		spin_unlock_irqrestore(&trans(priv)->reg_lock, reg_flags);
 		return;
 	}