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-mac80211.c b/drivers/net/wireless/iwlwifi/iwl-mac80211.c
index bdcbbd0..91a42c3 100644
--- a/drivers/net/wireless/iwlwifi/iwl-mac80211.c
+++ b/drivers/net/wireless/iwlwifi/iwl-mac80211.c
@@ -445,7 +445,7 @@
 	if (iwlagn_hw_valid_rtc_data_addr(base)) {
 		spin_lock_irqsave(&trans(priv)->reg_lock, flags);
 		ret = iwl_grab_nic_access_silent(trans(priv));
-		if (ret == 0) {
+		if (likely(ret == 0)) {
 			iwl_write32(trans(priv), HBUS_TARG_MEM_RADDR, base);
 			status = iwl_read32(trans(priv), HBUS_TARG_MEM_RDAT);
 			iwl_release_nic_access(trans(priv));