mv_xor: fix race in tasklet function

use mv_xor_slot_cleanup() instead of __mv_xor_slot_cleanup() as the former function
aquires the spin lock that needed to protect the drivers data.

Cc: <stable@kernel.org>
Signed-off-by: Saeed Bishara <saeed@marvell.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>