dma: use BUG_ON correctly in mv_xor.c, v4

This patch makes BUG_ON() usage correct in drivers/dma/mv_xor.c

Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Coly Li <bosong.ly@taobao.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>