Update CONFIG_MD_RAID6_PQ to CONFIG_RAID6_PQ in drivers/dma/iop-adma.c

Commit f5e70d0f renamed MD_RAID6_PQ to RAID6_PQ,
but iop-adma.c didn't update synchronously.

Signed-off-by: Wei Yongquan <weiyqlq@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>