soc: fsl: dpio: properly compute the consumer index

Mask the consumer index before using it. Without this, we would be
writing frame descriptors beyond the ring size supported by the QBMAN
block.

Fixes: 3b2abda7

 ("soc: fsl: dpio: Replace QMAN array mode with ring mode enqueue")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>