soc: fsl: qbman: read ithresh from HW

Read the DQRR interrupt threshold directly from the hardware.

Signed-off-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: Roy Pledge <roy.pledge@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>