ioat: cleanup some long deref chains and 80 column collisions

* reduce device->common. to dma-> in ioat_dma_{probe,remove,selftest}
* ioat_lookup_chan_by_index to ioat_chan_by_index
* multi-line function definitions
* ioat_desc_sw.async_tx to ioat_desc_sw.txd
* desc->txd. to tx-> in cleanup routine

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