IB/qib: Roll back PCIe tuning change

Commit 8d4548f2 ("IB/qib: Default some module parameters optimally")
introduced an issue with older root complexes.  They cannot handle the
pcie_caps of 0x51 (MaxReadReq 4096, MaxPayload=256).

A typical diagnostic in this situation reported by syslog contains
the text:

  [PCIe Poisoned TLP][Send DMA memory read]

Restore the module paramter default to zero with will avoid any
changes in the root complex.
Reviewed-by: Mark Debbage <mark.debbage@qlogic.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/qib/qib_pcie.c

@@ -560,7 +560,7 @@ static int qib_tune_pcie_coalesce(struct qib_devdata *dd)
  * BIOS may not set PCIe bus-utilization parameters for best performance.
  * Check and optionally adjust them to maximize our throughput.
  */
-static int qib_pcie_caps = 0x51;
+static int qib_pcie_caps;
 module_param_named(pcie_caps, qib_pcie_caps, int, S_IRUGO);
 MODULE_PARM_DESC(pcie_caps, "Max PCIe tuning: Payload (0..3), ReadReq (4..7)");