ioat: fix self test for multi-channel case

In the multiple device case we need to re-arm the completion and protect
against concurrent self-tests.  The printk from the test callback is
removed as it can arbitrarily delay completion of the test.

Cc: <stable@kernel.org>
Cc: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>