ioatdma: fix sed pool selection

The array to lookup the sed pool based on the number of sources
(pq16_idx_to_sedi) is 16 entries and expects a max source index.
However, we pass the total source count which runs off the end of the
array when src_cnt == 16.  The minimal fix is to just pass src_cnt-1,
but given we know the source count is > 8 we can just calculate the sed
pool by (src_cnt - 2) >> 3.

Cc: Dave Jiang <dave.jiang@intel.com>
Cc: <stable@vger.kernel.org>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>