RDMA/cxgb3: Limit fast register size based on T3 limitations

T3 firmware only supports one WRs worth of page list for fast register
work requests.  The driver currently allows 2 WRs worth, which
doesn't work for T3, so reduce the limit in the driver.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>