iop-adma: add a dummy read to flush next descriptor update

iop-adma: add a dummy read to flush next descriptor update

The current dummy read references the wrong address allowing the next
descriptor address update to linger in the store buffer and get passed
by an 'append' event.

This issue was uncovered by the change from strongly-ordered to device
memory for the adma registers.

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