ioat: fix ioat3_irq_reinit

The implementation of ioat3_irq_reinit has two bugs:

1/ The mode is incorrectly set to MSIX for the MSI case

2/ The 'dev_id' parameter to free_irq is the ioatdma_device not the channel in
   the msi and intx case

Include a small cleanup to clarify that ioat3_irq_reinit is only for bwd
hardware

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