soc: fsl: dpio: use an explicit NULL instead of 0

Use an explicit NULL pointer when calling qbman_swp_enqueue_multiple()
instead of a plain integer. Without this fix, we get the following
compile time error.

drivers/soc/fsl/dpio/dpio-service.c:466:60: warning: Using plain integer as NULL pointer

Fixes: 9d988097 ("soc: fsl: dpio: Adding QMAN multiple enqueue interface")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>