ioatdma: fix selection of 16 vs 8 source path

When performing continuations there are implied sources that need to be
added to the source count. Quoting dma_set_maxpq:

/* dma_maxpq - reduce maxpq in the face of continued operations
 * @dma - dma device with PQ capability
 * @flags - to check if DMA_PREP_CONTINUE and DMA_PREP_PQ_DISABLE_P are set
 *
 * When an engine does not support native continuation we need 3 extra
 * source slots to reuse P and Q with the following coefficients:
 * 1/ {00} * P : remove P from Q', but use it as a source for P'
 * 2/ {01} * Q : use Q to continue Q' calculation
 * 3/ {00} * Q : subtract Q from P' to cancel (2)
 *
 * In the case where P is disabled we only need 1 extra source:
 * 1/ {01} * Q : use Q to continue Q' calculation
 */

...fix the selection of the 16 source path to take these implied sources
into account.

Note this also kills the BUG_ON(src_cnt < 9) check in
__ioat3_prep_pq16_lock().  Besides not accounting for implied sources
the check is redundant given we already made the path selection.

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