iwlwifi: read the Rx write pointer only once

The FH (DMA engine) tells the driver the index of the last
ready (closed) Rx buffer. This data is in closed_rb_num.
If we read this data several times we may get inconsistencies
between the code and the debug prints which can make it
harder to debug issues here.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/iwlwifi/pcie/rx.c

@@ -698,7 +698,7 @@ static void iwl_pcie_rx_handle(struct iwl_trans *trans)
 
 	/* uCode's read index (stored in shared DRAM) indicates the last Rx
 	 * buffer that the driver may process (last buffer filled by ucode). */
-	r = le16_to_cpu(rxq->rb_stts->closed_rb_num) &  0x0FFF;
+	r = le16_to_cpu(ACCESS_ONCE(rxq->rb_stts->closed_rb_num)) & 0x0FFF;
 	i = rxq->read;
 
 	/* Rx interrupt, but nothing sent from uCode */