RDMA/cxgb4: Advertise ~0ULL as max MR size

Lustre uses a advertised max MR size of ~0ULL to indicate it should
use a dma_mr.  Hence advertise max MR size as ~0ULL.

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