RDMA/cxgb4: Max fastreg depth depends on DSGL support

The max depth of a fastreg mr depends on whether the device supports
DSGL or not.  So compute it dynamically based on the device support
and the module use_dsgl option.

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