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-tt.c b/drivers/net/wireless/iwlwifi/iwl-agn-tt.c
index 7712c55..9a12b70 100644
--- a/drivers/net/wireless/iwlwifi/iwl-agn-tt.c
+++ b/drivers/net/wireless/iwlwifi/iwl-agn-tt.c
@@ -189,7 +189,7 @@
 		}
 		iwl_read32(trans(priv), CSR_UCODE_DRV_GP1);
 		spin_lock_irqsave(&trans(priv)->reg_lock, flags);
-		if (!iwl_grab_nic_access(trans(priv)))
+		if (likely(iwl_grab_nic_access(trans(priv))))
 			iwl_release_nic_access(trans(priv));
 		spin_unlock_irqrestore(&trans(priv)->reg_lock, flags);