RDMA/nes: Mitigate compatibility issue regarding PCIe write credits

Under heavy load, there is an compatibility issue regarding PCIe write
credits with certain chipsets.  It can be mitigated by limiting read
requests to 256 Bytes.

This workaround is always enabled for Tbird2 on Gladius.  We also add
a module parameter to enable workaround for non-Gladius cards.

Signed-off-by: Chien Tung <chien.tin.tung@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>