GenWQE: Add wmb before DDCB is started

Needed to add wmb() before we send the DDCB for execution.
Without the syncronizing it failed on System p.

Signed-off-by: Frank Haverkamp <haver@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>