ioat: Fix some attach/detach issues

Don't run the selftest until after we've enabled bus mastering, or the
DMA engine can't copy anything for our test.

Create the ioat_test device on attach, if so tuned. Destroy the
ioat_test device on teardown.

Replace deprecated 'CALLOUT_MPSAFE' with correct '1' in callout_init().

Sponsored by: EMC / Isilon Storage Division