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>