ioat: prepare the code for ioat[12]_dma_chan split

Prepare the code for the conversion of the ioat2 linked-list-ring into a
native ring buffer.  After this conversion ioat2 channels will share
less of the ioat1 infrastructure, but there will still be places where
sharing is possible.  struct ioat_chan_common is created to house the
channel attributes that will remain common between ioat1 and ioat2
channels.

For every routine that accesses both common and hardware specific fields
the old unified 'ioat_chan' pointer is split into an 'ioat' and  'chan'
pointer.  Where 'chan' references common fields and 'ioat' the
hardware/version specific.

[ Impact: pure structure member movement/variable renames, no logic changes ]

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