IB/mthca: fix posting of send lists of length >= 255 on mem-free HCAs

On mem-free HCAs, when posting a long list of send requests, a
doorbell must be rung every 255 requests.  Add code to handle this.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>
diff --git a/drivers/infiniband/hw/mthca/mthca_wqe.h b/drivers/infiniband/hw/mthca/mthca_wqe.h
index 73f1c0b..e7d2c1e 100644
--- a/drivers/infiniband/hw/mthca/mthca_wqe.h
+++ b/drivers/infiniband/hw/mthca/mthca_wqe.h
@@ -50,7 +50,8 @@
 
 enum {
 	MTHCA_INVAL_LKEY			= 0x100,
-	MTHCA_TAVOR_MAX_WQES_PER_RECV_DB	= 256
+	MTHCA_TAVOR_MAX_WQES_PER_RECV_DB	= 256,
+	MTHCA_ARBEL_MAX_WQES_PER_SEND_DB	= 255
 };
 
 struct mthca_next_seg {