dma: mv_xor: Remove unneeded NULL address check

This mmio address is checked at probe-time, which makes this test
redundant. Let's just remove it.

Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>