dmaengine: mv_xor: fix oops when channels fail to initialise

When a channel fails to initialise, we error out and clean up any
previously unregistered channels by walking the entire xordev->channels
array.  Unfortunately, there are paths which end up storing an error
pointer in this array, which we then try and dereference in the cleanup
code, which causes an oops.

Fix this by avoiding writing invalid pointers to this array in the first
place.

Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>