qcacmn: Replenish more RX buffers when refill ring runs low

When nbuf allocation failure happens, there is no retry scheme.
So RX buffers in refill ring may shrink and not got enlarged.
This change is aimed to replenish more RX buffers when watermark
is less than critical low threshold.

Change-Id: I201b9e252ba08ba1bff47e0b5ec819a45f1b1ddf
CRs-Fixed: 3245915

dp/wifi3.0/dp_rx.h

@@ -1482,6 +1482,7 @@ dp_rx_update_flow_tag(struct dp_soc *soc, struct dp_vdev *vdev,
 }
 #endif /* WLAN_SUPPORT_RX_FLOW_TAG */
 
+#define CRITICAL_BUFFER_THRESHOLD	64
 /*
  * dp_rx_buffers_replenish() - replenish rxdma ring with rx nbufs
  *			       called during dp rx initialization