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: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>