RDMA/cxgb4: Always do GTS write if cidx_inc == CIDXINC_MASK

When polling, we do a GTS update if the accumulated cidx_inc == the CQ
depth / 16.  However, if the CQ is large enough, Cq depth / 16 exceeds
the size of the field in the GTS word.  So we also need to update if
cidx_inc hits CIDXINC_MASK to avoid overflowing the field.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>