iw_cxgb4: set correct FetchBurstMax for QPs

The current QP FetchBurstMax value is 256B, which
is incorrect since a WR can exceed that value.  The
result being a partial WR fetched by hardware, and
a fatal "bad WR" error posted by the SGE.

So bump the FetchBurstMax to 512B.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>